What kind of switch takes a 12v in sends it to one of two output wires?
If you do Google Images "SPDT" you'll get a huge list of circuit diagrams and photographs that might help clarify things.  Not really a lot else to say.  They're named like this:-



Apart from one thing.  Make sure that the current rating of the switch (and wires) is adequate.  Don't use a little logic level switch to change between the main battery and the additional battery for the 2T winch.  Any concerns, ask here...