The Ki power Flurry of Blows, is very simple:
Immediately after you take the Attack action on your turn, you can spend 1 ki point to make two unarmed strikes as a bonus action.
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 you hit a creature with one of the attacks granted by your Flurry of Blows, you can impose one of the following effects on that target.
"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.