Only when the caster is below 5th level
The errata for the PHB has clarified the restriction further from the original printing:
To be eligible for Twinned Spell, a spell must be incapable of targeting more than one creature at the spell’s current level.
By default, eldritch blast does not have a range of self and is capable of targeting only one creature. It is thus eligible to be twinned.
However, the spell becomes capable of targeting more creatures once the caster reaches level 5:
The spell creates more than one beam when you reach higher levels: two beams at 5th level, three beams at 11th level, and four beams at 17th level. You can direct the beams at the same target or at different ones. Make a separate attack roll for each beam.
Thus, once the caster reaches 5th level and above, they can no longer twin eldritch blast.
Note that eldritch blast (and all other similar cantrips) scale with character level not class level as confirmed (unofficially) by Jeremy Crawford on Twitter. (See Do Cantrips use your character level or class level? for more discussion about that)
Yes, Simulacrum can be twinned
It creates one duplicate each of two creatures.
Just to take the first line of the Simulacrum spell:
You shape an illusory duplicate of one beast or humanoid that is within range for the entire casting time of the spell.
The target is one beast or humanoid within range touch.
There's nothing logically stopping this from working. Simulacrum doesn't target self or more than one creature.
And to pull out a line of the Twinned Spell feature:
[snip] ... target a second creature in range ... [snip]
If you had two people (one of which could be yourself) within the range of "touch" for 12 hours and you have the sorcery points (you'll need seven), then you should be able to twin the spell and create two duplicates: one of each creature.
Also as a restriction of Twinned Spell, a twinned simulacrum can not be used to create two duplicates of the same creature.
So, to answer your questions succinctly:
- No, a twinned simulacrum must target different creatures.
- Yes.
Best Answer
When twinning a spell that uses an attack roll, you roll two attack rolls and two separate damage rolls.
When twinning a spell that does not use an attack roll, you roll damage once.
The general rule for when a spell deals damage to multiple targets at the same time is as follows:
Twinning a spell causes the same spell instance to get a new target, so normally this would be the rule that applies when determining whether or not to roll damage multiple times. Something like a twinned Poison Spray would have a single damage roll that covers both targets as a result.
For spell attacks, however, there is a more specific rule that determines when you roll damage. (Emphasis mine)
The rules for making an attack are specific that when you make the attack roll, if you hit, you roll damage. Since you're making a separate attack roll for each target, you roll damage separately for each hit as per step 3 in the quoted section. (You could also argue that these separate attack rolls mean you're not dealing damage to the two targets at the same time, so the earlier rule on hitting multiple targets at the same time wouldn't apply anyways)
Worth noting that while the initial attack of a twinned Witch Bolt would be two attack rolls and two damage rolls, Witch Bolt's optional damage on the following turns would be one roll for both targets since it no longer requires you to make an attack roll to deal the followup damage.