Voss doesn't set a number of targets, neither does Kaargra. They only require that a target meet the HP requirement to receive the damage.
They deal two instances of damage and do not require 2 targets be hit.
Siege Breaker hits the 2 targets with the highest HP, it cannot hit the same target twice.
Whether or not the way the app currently looks like it works is the right way, or if it just doesn't matter yet because nothing can add a hero target mid villain damage right now, is to be seen.
It also could be programmed to account for new targets if they come into play and we just haven't seen that part work, it will never be such a common event that normal play should predicate it being resolved.
The main thing with the app right now is that it is important that the app determines the targets before dealing damage, because in a case like Siege Breaker, you can resolve damage against the second highest first, and then resolve damage against the highest. So it must determine all valid targets before asking you which order to resolve in.
Even if a new target were added mid-damage, it would need to establish the current 2 highest hp targets before the first damage can be dealt
EDIT:
Actually there is a way to test this. Cyrus Hayes, Bee Bot, forced deployment. Have no other 3 or fewer hp targets in play.
You should have to choose either Bee Bot or a 4+hp target to deal first damage to, choose Bee Bot, destroy forced Deployment, see if you get the option.
Again this might reveal the official ruling, or something the app isn't prepared to handle yet.