Sounds like speaker A is more efficient than speaker B. That's to say that, given the same amount of power, one speaker will play louder than the other.
Short of putting volume limiting level controls on speaker A*, there's really not too much you can do.
If it's an AVR with zone 2 outputs, you could feed their line-level outputs to a separate integrated amp and use that to control the volume to speaker B.
*and that's not a really good way to do things. However, if speaker B were louder, it would be.