Larry wrote in :

Simply adding a big paralleling switch (or a new solenoid even more
appropriate) to put all the battery banks in parallel when any engine is
running will solve the problem without tearing out all the wiring. In
parallel, it doesn't make any difference which alternator or both is
charging...works fine.



oops....reread your post and it says the solenoid is there. Are you SURE
they are in parallel when EITHER engine is running?


--
Larry