Duel battery ops for my F1115?

[bullfighter18]

I would like to add a second battery to my system. Can both (-) be tied to eachother and then one (-) lead from 1 battery be grounded to the engine block? What guage wire would I use for this tie and grounding? Thanks, chris

[boats.net]

Connect engine negative battery cable to first battery and connect a jumper wire between negative post of battery one and battery 2, connect engine positive battery cable to common post on battery switch, connect wire to battery 1 post on switch to battery 1 positive post and battery 2 post on switch to battery 2 positive post. 4 Gauge will be ok if wires only need to be a couple feet long.