In searching for answers to this, I've come across an interesting point on the Paizo forums. If you are in one of the far corners and you consider these not threatened, then you can diagonally move towards the character and never provoke an AOO (that is, per RAW)... and yet, logically, a threatened area should make an uninterrupted circle around the creature. This may explain why 3.5 made an exception out of this.
Yes, that is precisely why 3.5e made that exception, and it’s also why Paizo issued an official FAQ that changed Pathfinder’s rules to add the same exception, as @caps reports in this fine answer that you should go upvote.
Thus, a reach weapon can attack the following \$X\$’s from \$C\$:
\begin{array}{c|c}
\phantom{X} & \phantom{X} & \phantom{X} & \phantom{X} & \phantom{X} & \phantom{X} & \phantom{X} \\ \hline
& X & X & X & X & X & \\ \hline
& X & & & & X & \\ \hline
& X & & C & & X & \\ \hline
& X & & & & X & \\ \hline
& X & X & X & X & X & \\ \hline
\\
\end{array}
Before the FAQ change
However, the FAQ entry that caps reports did not exist at the time this question was asked. At that time, you did not get the four corners, and so could only attack these \$X\$’s from \$C\$:
\begin{array}{c|c}
\phantom{X} & \phantom{X} & \phantom{X} & \phantom{X} & \phantom{X} & \phantom{X} & \phantom{X} \\ \hline
& A & X & X & X & & \\ \hline
& X & B & & & X & \\ \hline
& X & & C & & X & \\ \hline
& X & & & & X & \\ \hline
& & X & X & X & & \\ \hline
\\
\end{array}
Here, it would appear that someone could step from \$A\$ to \$B\$ to avoid an attack of opportunity altogether. However, even before the FAQ just changed this to be like 3.5e, the developers at Paizo had ...for lack of a better word, we’ll call it a clarification, though it honestly just confused me more. From an earlier FAQ:
Can you or can you not attack diagonally at a distance of 2x squares (15'=10' exception) with a reach weapon?
James Jacobs: Nope. A reach weapon gives a specific extension to your reach. When you count out squares, since every other square is doubled when you count diagonally, that means that there’ll be corners where you can’t reach.
Sean K. Reynolds: It's an artifact of the grid. The closest the rules come to addressing this is in Large, Huge, Gargantuan, and Colossal Creatures, which says:
Unlike when someone uses a reach weapon, a creature with greater than normal natural reach (more than 5 feet) still threatens squares adjacent to it. A creature with greater than normal natural reach usually gets an attack of opportunity against you if you approach it, because you must enter and move within the range of its reach before you can attack it.
So just because the grid has a square for "15 feet away" and a square for "5 feet away," but no square for "10 feet away," using that corner path doesn't mean you're magically teleporting from 15 feet to 5 feet; you are passing through a 10-foot-radius band around the creature, and therefore you provoke an AOO.
Admittedly it's not clear, and obviously it doesn't have the diagram in the 3E book to provide a non-textual example, but it's supposed to work as I described above.
Basically, the idea was, under the rules at the time, you didn’t threaten 15 ft. away, so you don’t get the corner, but you did threaten 10 ft. away and there’s no way to move from 15 ft. away to 5 ft. away without passing through a point that is 10 ft. away. Thus, someone moving from 15 ft. away on the diagonal to 5 ft. away on the same diagonal was going to provoke even under these rules.
So the enemy at \$A\$ moving to the point marked \$B\$ towards \$C\$ with a reach weapon provoked an attack of opportunity (assuming this isn’t a 5 ft. step of course), because somewhere between \$A\$ and \$B\$, there is a point that is 10 ft. away from \$C\$ that the enemy has to pass through.
Presumably, you would have adjudicated the enemy’s position for the purposes of the attack of opportunity as being \$A\$, though this was never made clear. In this sense, the end result was identical to the 3.5e/post-FAQ version for movement towards you: creatures leaving that corner square to enter a square inside your reach provoked an attack of opportunity as if you threatened that square. You were not eligible to make an attack of opportunity if the enemy performs any other action that provokes from \$A\$, including movement in other directions, because you do not actually threaten it.
This was a headache. Even before the FAQ changed things to match 3.5e, that was precisely what I recommended:
Reach weapons are one of the few fairly-nice things that melee can get. There’s really no need to nerf them. I strongly suggest that you straight-up ignore this nonsense and use the 3.5 rule. The exception to the usual calculation of ranges in the case of reach weapons is weird, but clearly there was a good reason for it: without it, you wind up with this mess.
Sorry, one movement only provokes one AoO from a given defender.
See the d20pfsrd:
If you have the Combat Reflexes feat, you can add your Dexterity modifier to the number of attacks of opportunity you can make in a round. This feat does not let you make more than one attack for a given opportunity, but if the same opponent provokes two attacks of opportunity from you, you could make two separate attacks of opportunity (since each one represents a different opportunity). Moving out of more than one square threatened by the same opponent in the same round doesn't count as more than one opportunity for that opponent. All these attacks are at your full normal attack bonus.
So if you shriek "SERPENTINE!!!" and run circles around your opponent, they still only get one "leaving a threatened square" AoO on you.
The opportunity from Hold the Line is different, but only works once upon them entering your threatened area. So you would get only one from Hold the Line and one for them leaving a threatened square.
Note that the leaving a threatened square is per round - not even just per turn - with the Mythic rules in use in our WotR campaign we've been finding a lot of cases where you get to act multiple times in a round, even charge multiple times, etc. If by hook or crook someone charged you, ran off, charged you again, and ran off again over the course of a round (on their turn or across multiple turns), you would still only get 1 AoO for them leaving your threat but could get multiple from Hold the Line, if they actually leave and enter your threatened area with a charge twice. Up to the limit of the number of AoOs you have.
Best Answer
When they move over 5ft and 10ft away from you
You can use your one reaction to execute an opportunity attack to attack with the dagger if the opponent steps out of 5 foot reach. Or the Whip if the opponent steps out of 10 feet reach.
This is supported by the descriptions of the reach property (post-PHB-errata) which says:
But you only get one or the other as you only get one reaction until after your next turn.
Two-weapon fighting
Since a whip is not a light weapon you can't use two-weapon fighting with a dagger.
But that only relevant to when you are taking your turn. The rule is limited to a grant of a bonus action to attack with the second light weapon. There is no rule prohibiting you from holding any combination of two one handed weapons. Without both of them being light you can only attack with one or the other with an attack option.
Also note that you only get one bonus action regardless of how many you could have.
In my opinion the main advantage of using two one-handed weapons without being able to use two-weapon fighting is to take advantage of some special property on one weapon versus the other. For example the use of a whip and a longsword. The whip has reach but less damage. So close in you use the longsword but if you need to attack an opponent further away you can attack with the whip. And it expands you flexibility in using a reaction for taking an opportunity attack.
Rules Details
From page 69 of the Players Rules for Basic D&D 5e.
From page 74 of the Players Rules for Basic D&D 5e.
and
From page 70 of the Players Rules for Basic D&D 5e.