The best way is to run EQUAL lenghts and guage of cable from both the + and - terminals on both batteries to a central block or point you can connect your two 12v wires to.

batt 1 (+) (-)
l l
l l
central connect O------ O-------- l l l l
l l l l
batt 2 (+) l (-) l
l l
l l
l l
boat 12v + wire- l l- boat 12v - wire
l l
(+) O (-) O