Connecting batteries to pressure washer to charge them while pressure washer is on

I have a 8gpm pressurePro with a electric start engine which charges the battery while in use. I was wondering if anyone knows if I could connect my roof pump battery to this battery in series or parallel so that both batteries get charged together by the PW?

I have a second battery mounted to my PW trailer to run a small pump that feeds a small pw for rinsing. The service shop I take my machine too every now and again when I just can’t figure out what is wrong- said I all I have to do is splice “this particular wire” that he pointed out that comes off the alternator and run it to my other battery so I wont have to put a charger on it. So if you can get the wiring schematic or an engine guy knowledgeable enough to point out that wire, you can just run that to your battery for a trickle charge just like the main battery gets. He said since it’s a trickle charge it wouldn’t effect the charge of the main battery enough to have 2 hooked up.

I’m not an electrician, but I think I asked him about running parallel and he said it would be too many volts going to either the pump or the main battery. So I opted for the wire splice. No problems so far and batteries stay charged!