Flurry of Blows modifies your full-attack
It adds additional attacks, and applies a penalty. It is otherwise a normal full-attack like any other.
This means you may use any weapon you have available for any given attack. If you are holding two weapons, you can use one or the other for each attack you’d normally have.
Note that this is exactly the same as when you don’t use Flurry of Blows. If you have multiple attacks for whatever reason (BAB +6 or higher, the haste spell, whatever), you can use any weapon you have available for any given attack as you please.
On the flip side, you do not have any more attacks, normally, than you would with a single weapon. You simply get the attacks you would otherwise have. Those can be the ordinary attacks of your full-attack, or extra attacks from Flurry of Blows, or whatever. Simply having two weapons does not give you extra attacks.
To get extra attacks, you have to use the Two-Weapon Fighting combat option (and then you are subject to its penalties and restrictions). In particular, Two-Weapon Fighting is an exception to this “any weapon for any attack” bit. Note that this is more than just having two weapons; you have to actually choose to use Two-Weapon Fighting, or none of the following applies. When you do choose to use Two-Weapon Fighting, it changes your full-attack (much as Flurry of Blows does): it grants an extra attack and applies a penalty, and it also applies a special restriction on which weapon you can use for which attack: specifically, your bonus attack from Two-Weapon Fighting has to be a different weapon than the one you are otherwise using.
Furthermore, under Pathfinder rules, Flurry of Blows is Two-Weapon Fighting (except it doesn’t actually require you to use two weapons). As such, the idea is you cannot use it and Two-Weapon Fighting at the same time. If this is the case, there is absolutely zero benefit to having two weapons, except I guess the ability to pick which one you want. You’re better off with a more-powerful single weapon than splitting your wealth between two.
Unfortunately, the rules here are notoriously vague and have required numerous amendments, explanations, Paizo contradicting themselves several times, and finally overruling previous rules entirely because they never made sense. This is actually one of the most famous fiascoes in Pathfinder history, and the rules are left a mess as a result. I suggest just returning to the 3.5 rules, before all this started: Flurry of Blows and Two-Weapon Fighting are completely separate things, combine them if you want (taking even more penalties and gaining even more attacks). This is still a bad idea, since it is a very weak option, but at least the rules are clear, make sense, and there’s actually, theoretically, a benefit to dual-wielding as a monk.
Best Answer
The Ki power Flurry of Blows, is very simple:
Per Melee Attacks, an "unarmed strike" is a "melee weapon attack". Making an Attack includes three discrete steps, the attack roll is one of them.
Unless it says otherwise, multiple attacks are just that - multiple attacks, not a single attack that deals damage more than once. If there aren't two attack rolls, it isn't two attacks - it's one. Each attack must be rolled separately and hits or misses on it's own - anything that triggers off the hit (such as damage) is likewise resolved on its own. Extra Attack works the same way.
Way of the Open Hand's Open Hand Technique says:
"Whenever" and "one of the" are the key bits of phrasing here. The two attacks don't have any effect on each other. If both attacks hit, you can inflict a different effect for each hit. In fact, you should resolve them individually and sequentially. If you knock the target prone, you have advantage on the next attack. If you knock the target away with the first attack, you need to use movement to make the second.