unless I have really missed something, it seem to me that when both FET's are on, the lower battery is basically shorted by the 2 FET's.
also the gate for the FET on the right needs to be higher by whatever the lower battery voltage is. IE, if the FET needs 5 volt to be completely on, non-linear, and the lower battery is 12 volts, then you will need to have 12+5=17 volts at the gate to get it out of its linear region. If its only partially on, then its dissipating part of the power, and if the load is a fairly small resistance, it could be basically dissipating the majority of the load. Yes it will get hot then, and quick.