power distribution between subs?

It's basic electricity.  Kirchhoff's current law states that the current into any electrical node (connection) will always equal the current out.  Meaning, if the amplifier is capable of producing, say, 10 amps of current, this current will be split at the node where the speakers are connected such that the total still equals 10.  If there are two speakers both at the same impedance, they will each use 5 amps, or "split" the available power.  If one has higher impedance than the other, it will utilize less of the current but the total will still equal 10 amps.  You cannot create more current (or more power) than the source is capable of supplying.

The only reason there is any question about fundamental issues like this is because too many people who think of themselves as car audio "experts" have no training in basic electricity or electronics and really do not understand what they are talking about.