It would only do 10 damage, not 15. Each target will only get hit once per spell cast.
However, if you target your opponent with your first bolt then a creature with the second, the opponent would take 10 damage and the creature would take 5.
I.e. in order to see the added effect, you have to choose a unique target each time.
PU is a single target spell, so it would work... Of course your opponent could pull the same trick to get their own copies. They could also zap all of your PU'd creatures with a single bolt or other CC. (AM would be particularly nasty).
It certainly opens up some heavy damage combo's... but so do fractal and sky-blitz, which are similarly balanced by high cost and total pool drain.