"means usually getting the big one down to 50% while there is still lots left in the inverter's bank"
Certainly a 50% discharge is talked about mucho on this forum but its just a convienent number. Trojan for instance says on their Signature Flooded Line including T105, T125 and T145 that if you discharge to 50% you can expect 1,250 cycles but if you only discharge to 60% you can expect 1,500 cycles....almost an extra year of camping. I'd keep them separate, protect the most expensive battery first and switch when you get to a 60% SOC.