You typically do not want to do that with batteries of different sizes. Will it blow anything up? Probably not, and it’s probably *less* of a deal since batt types are lead acid. But it’s not what’s recommended. Usually you want to isolate each battery via the use of diodes.
Do some research on “dual-battery systems”...typically for boats.
I saw that discussed in a couple places, and another FReeper mentioned that upthread.
Gotta duck out now for some sleep, appreciate the feedback.