I agree with big tee and red 58. Ive done it the same way many times,,w/no probs.
I had an old forklift with the set-up as Chris(Mt Tam Trucker)talked about. Its called an Orpin Switch,that ties the two batteries together. That worked fine also.
If your 6v system is in good shape,IE: good,clean grounds,and terminals,proper size cables,all connections are corrosion free,
motor tuned right,charging system dialed in,, you should have no starting problems.
Good Luck,
Marty
