Using a ford relay what is the wiring diagram to connect two batterys

1976 Ford F-150

Hook em up positive to positive, and negative to negative, naturally.

I would pull power from the center of the connector between the two positive terminals, so as to not overwork one battery.

If you want switching capacity from one to the other, then you can install a marine battery switch to do this, and the directions are on the switch. Just get one that will allow "one", "two", "both", "off" and you'll be in good shape. Most switches say they are NOT to be used while the equipment is running, as some of them will blow diodes if they are switched while the motor is running.

regards, P