Numenera is a game that is fairly resistant to the effects of party imbalance, so it isn't too much of a problem once your players realize that experience spent on short-term benefits can mean the difference between life and death (or at least unconsciousness), and you realize that setting difficult encounters can sometimes be a good thing.
You will learn how best to balance combat for your party as you go, and while some combats should be a breeze and others are challenging but not overly difficult, others should be difficult to the point where they do have the potential to lose, and short-term experience spending may be (part of) the key to winning. (On a similar note, this is also important for promoting the use of cyphers and artifacts because your players will more than likely begin the game hoarding them like treasure).
If you still feel the need to house-rule experience expenditure after playing for a while, solution 1 is the better of your two ideas, and has been put forward by Monte Cook himself as an optional way of handling experience in the Numenera Design Diaries (under the section Using XP - fourth paragraph). The only real disadvantage to this system is that it does limit the way your players can spend their experience, but it also makes the decision of whether to spend that last point of short-term experience that much more critical.
Your second idea would require you to halve the experience you give out since each point would be used twice. Your players may also spends their experience on short-term benefits they don't need just so they can push the experience to the long-term pool to be used on character advancement all the sooner.
From the margin notes of p26 of the Cypher System Rulebook:
Might defense: Used for resisting poison, disease, and anything else
that can be overcome with strength and health.
Speed defense: Used for dodging attacks and escaping danger. This is
by far the most commonly used defense task.
Intellect defense: Used for fending off mental attacks or anything
that might affect or influence one’s mind.
The comparison for Might Defense would seem more about endurance and resilience, than your expected interpretation. Just as Intellect Defense is more about willpower and prodigious psychic fortitude.
Further detail in the Rules of the Game reinforces this differentiation, per the Action: Defend sub-section (CSR p209) -
The type of defense roll depends on the type of attack. If a foe
attacks a character with an axe, she can use Speed to duck or block it
with what she’s holding. If she’s struck by a poisoned dart, she can
use a Might action to resist its effects. If a psiworm attempts to
control her mind, she can use Intellect to fend off the intrusion.
This is not to say that you cannot parry with an object, take advantage from a cluttered environment or heft a barricade against a pummeling assault. However, here you need to grasp the nuances of the system. You still use a Speed Defense to square off against an attacker, intercede with a shield at the right moment or brace against an assault at the right moment, but you will reduce your difficulty with appropriate skills - like the faculty to take advantage of your environment - or assets - like a keenly fashioned shield, a neural-linked cybernetic hand or a well-braced barricade of lashed together timbers.
Example of Two Warriors - one of Speed, one of Might
See the cartoon strip The Red Viper Vs. The Mountain That Rides for the nature of the battle. If you're a fan of Game of Thrones who hasn't read A Storm of Swords or seen all of Season 4, and doesn't want a spoiler - STOP HERE!
Oberon Martell is a Vengeful Warrior (Might Edge, trained in Speed Defense) who Moves Like A Cat, with Extra Edge (Speed), Trained Without Armor (Speed Defense trained), Thrust (+1 damage with a stabbing weapon, c: 1 Might) and Fleet of Foot (succeed at d2 Speed roll to run, taking a move and action in the same round). M11, S16, I10. Oberon is armed with a light spear (Light Weapon).
The Mountain is a Tough Warrior (Might Edge, trained in Might Defense) who Masters Weaponry (+1 point of dmg with any weapon), with Control the Field (inflict 1 less damage, maneuver enemy into position you desire within immediate range), Practiced in Armor (starts with Heavy Armor, -3 dmg), No Need for Weapons (unarmed attack counts as Medium Weapon, doing 4 dmg) and Surging Confidence (when you use your first Recovery Roll of the day, you immediately gain another action, c: 1 Might). M16, S10, I8. The Mountain is armed with a two-handed sword (Heavy Weapon).
Oberon's player has put more of his 6-point pool into Speed than Might as well as getting +3 Speed from his Focus, while The Mountain has put all 6 points into Might.
Oberon uses his lightness of step and quick maneuvers to run rings around The Mountain. He trades the small amount of damage he manages to inflict for effects - specifically he looks to distract, knock back and move past his opponent.
Oberon goads The Mountain, tormenting him about the fate of his sister and her children at the hands of the monstrous warrior. Oberon might strike time and again - because of his high Speed and the lumbering armor-laden Mountain - but he damages primarily through rolling high.
The Mountain has his massive sword and hits infrequently because of his low Speed, but when he does hit he strikes for 7 damage or more. He will tend to use his Effort to inflicting more damage rather than decrease difficulty - and may trade these strikes of 10 damage to knockdown, impair or stun his opponent.
The Mountain doesn't need Speed to win. Indeed, he can't. He simply can't maneuver fast enough to defend himself. He lumbers and suffers a withering assault of small blows, but his armor soaks up much of it. He might have a two-handed sword so heavy no one else in the arena could lift it, but it can't parry or defend against the flurry of stabs from Oberon's spear. To parry, block or dodge means being in the right space at the right time - Speed Defense every time... but, The Mountain does not fight with Speed.
Oberon manages to poke through chinks in his armor, but Might Defense means The Mountain is never knocked back and hardly seems to feel anything. He isn't fast - he doesn't need to be. When he rolls 17 or higher, his attacks become massive - up to 14 points - and in the process of his raging swings, these blows kill members of the crowd outright.
In the end, he stuns Oberon after recovering points with his Surging Confidence, then uses his No Need for Weapons to poke out the fleet warriors eyes before rolling a 20 and squeezing out his brains with his bare hands.
Best Answer
Numenera is not more strongly tied to party size than any other trad style RPG. Like any trad style RPG, however, it's going to have trouble, or at least require special techniques, when dealing with, in this case, less than three or more than six characters.
Why Less Than Three?
There are three character classes - glaives, jacks, and nanos. In class based systems, unless the GM is specifically constructing adventures to avoid one of the primary concerns, you will often find yourself needing party members with those complementary skills.
Furthermore, in any game with much combat at all, especially when randomizers are in use, you have the "N+1 redundancy" problem. One party member being taken out by an unlucky roll - if there's one character, they're screwed; if there's two, it's still mighty hard for one person to haul another away especially if there's action going on (and what if no one else can heal...).
Of course, the Numenera book has a big chip on its shoulder in the GM advice section and strongly encourages you to tune challenges to the party you have (kinda) and to use fiat when necessary (unless you should be using the dice)... It's a little schizophrenic but the bottom line is "you can craft a campaign that is suited to fewer/non-balanced groups" but that takes a little extra zing on the part of the GM.
Why More Than Six?
Despite claims to the contrary, every tabletop RPG begins to break down with more than six players. Basic human group dynamics form a large portion of why (optimal team size is often cited at anywhere in that range, from 4.6 to "5-7" (which would include your GM).
Since Numenera combat is light, it can go up to six safely (something like Pathfinder or D&D 3.5e or 4e becomes a boring PITA the more people you have) but it still has an action economy, and the more PCs there are, the harder it is to make a meaningful challenge just because you have N "bites at the apple" for whatever challenge is at hand, and the higher N is the lower the chance of failure is, so you have to add larger and more diverse challenges which is doable but is more work.
So Numenera is more tolerant of fewer or more PCs than some games, but as a trad game with defined class roles and action-based combat, there is a natural sweet spot in the 3-6 range where better GMing and adaptation is required the farther outside that range you go.