The first guy should also have an interlock but you didn't say.

how I would want it run would be
3rd panel with an interlock between the genset and main panel.
Everything on the 3rd can be run by the genset.
1 of the mains feeds the 3rd panel and you can't have both the genset and mains powered on the 3rd panel. Ie the interlock on the 3rd panel.
the other 2 mains stay on city power full time.
a few reasons.
only 1 interlock to deal with.
no mistakes on leaving too many things hooked to the genset.
can tell when city power is back on.