I'd connect pos on bat 2 (under gen panel) first, then connect interconnect cable from neg on batt 2 to pos on bat 1 (under airbox) then Finally, connect neg on batt 1, which goes to ground. Never work on either battery without disconnecting Batt 1 neg to ground. I have a cut off switch at this point - neither battery connected to ground, so no worry if accidentally shorting to chassis with a spanner whilst tightening up the clamps.