Maybe not the answer you want but you can also rewire it... make the 60AH battery your starting battery and one near the headlamp the 100AH aux battery.
This is one of the things I've considered doing.
I think i've come to the conclusion I'm going to run a LiFEPO4 in the back. Sucks to have the system take up space, but keeps weight off the already heavy nose and probably the only way to get the usable 80 Ah I'm looking for. Two 50 Ah batteries can be tucked away pretty well near the wheel arches.
This is the other route I'm leaning towards. Cost is the biggest impediment at this point for a 100ah lithium. My solar charge controller is already in the cargo area, so easy to connect. Much more capacity then my 100 ah Group 31 AGM, which is about half the capacity.