Do-able, but "permanent" voltage drop issues should be avoided, more than temporarily issues when the genny is running at the new house. Plus if the old house becomes an office, with tax write off "business expenses", keeping the pump on it's meter may have advantages.
As far as one genny feeding power to 2 residences, one has to pay attention to the fact neutral & grounds are bonded at each residence's panel, and you don't want your genny circuit connecting the 2 neutral/ ground systems. i.e. you'd have 1 connected system (when in "emergency" mode) with 2 ground/neutral bond locations. Ground and neutral should only be bonded at 1 location.
Thus you want your transfer switch at the old house to be 3 pole (2 hots, 1 neutral) to not only transfer the circuit(s) power at the old house, but to also transfer the circuit(s) neutral of the genny powered loads to the neutral from the genny (which is bonded to ground at the new house's panel). The transfer switch at the new house would NOT switch the neutral. That is: the transfer switch in the old house should isolate and switch the neutral of the pump (and other genny powered loads) from the old house panel's neutral bus and connect them to the neutral coming from the genny (and new house) when in emergency mode.
Seems like you'd want the transfer switch/panel in the old house to be able to run the heat (controls) as well as the pump. and maybe a few light/power circuits. Load management may be required.