Kinda the same scenario i just did. I was told yes this can be done. Wire the cranking batteries together in parallel Lets call them A and B. Then take your charging lead and put the Pos lead to+ on battery A then the Neg lead to - on battery B
This is the correct way to do it. Just be sure your jumper wires are big enough to handle the amperage and use fine stranded wire. Cj
2002 X19 200HP OX66 HO Vmax,HPDI lower, it lives, thanks Hydro Tec.