Hi Kevin,
Its simple enough and I’ve done a diagram for you, but because this editor is not very friendly, I can’t attach it to this reply ! I’ve asked for help on this before but no one has come forward with how to do it.
So I will try to describe how to in text !
Have the 2 x 6v batteries in front of you and wire the POS+ of one battery to the NEG- of the other and thats it. The remaining two outside terminals +- will now give 12v to your motor. For the 6v simply take a lead from both terminals of one battery only +- will give you the 6v tap you require.
What you’ve done here is to wire the batteries in series – doubling the voltage but not the amperage – ie. 2 x 6v7a = 1 x 12v7a
Hope this helps without the diagram
Cheers – Kenny