Most people just combine the batteries to 24v and use a stick hand piece.I would also like to know how to weld with my dual-batteries, if I even can.
I can't stick weld to save myself so bought one of these
I've used is a lot in the bush and although gasless MIG welding is not my favourite it does do the job.
Welding a fellow camper's boat trailer, the Kimberley.
Welding my shockie mount, the small gennie is for a grinder. Simpson Desert.
The two batteries you can (just) see in the tray are normally wired for 12v, but I rearrange the cables to get 24v for welding.