In a parallel circuit, voltage stays the same across both loads but current drops. You are right that two 10 ohm loads in parallel presents itself as a 5 ohm load. Each speaker would have half the current flowing through it (i.e., the total current load is what goes through the first speaker plus the second).
Putting two dissimilar speakers together could sound funky and mess up the soundstage plus you could definately get peaks in frequency response since the cross-overs are not matched. Who knows -- maybe those peaks will compensate perfectly for the room acoustics. Chances are, it won't sound too good. But there is a very slim chance that it will. As long as the combined impedence isn't too low (i.e., don't put two 4 ohm loads in parallel), you could always give it a shot and see what happens.
Putting two dissimilar speakers together could sound funky and mess up the soundstage plus you could definately get peaks in frequency response since the cross-overs are not matched. Who knows -- maybe those peaks will compensate perfectly for the room acoustics. Chances are, it won't sound too good. But there is a very slim chance that it will. As long as the combined impedence isn't too low (i.e., don't put two 4 ohm loads in parallel), you could always give it a shot and see what happens.