Are you allowed to stack the feats of Power Attack and Combat Expertise in a single round?
Both penalties are untyped, and therefore stack with each other. You may have a hard time hitting anything, though!
Yes, but the penalties also stack, so it usually isn't done.
I did this with a critfisher. You hit if you roll within the 15-20 you crit at. So it doesn't matter what your to hit is. Attacked with +0/+0/-5/-5 at level 10 and still came out on top