6 volt vs 12 volt batteries

No.

No, only the voltage will double, the amp hours will remain the same as stated by the battery manufacturer. l hope you have room for 2 batteries where one would work OK.

Sam,
2 6v batteries together will give you 12v with the amp hour rating the same. I have just purchased 2 x 6v 275amp hour batteries for my caravan. So I will have 12v @ 275a/h. The 2 6v batts are lighter than 2 12v batts with the same amp hour capacity(in my case) and I got a really good deal. Both delivered to my house for $460. Local battery place could go to $600 and when I told them the price they got me to show them on the net. Then they rang them up and after that they said buy em. We cant get them anywhere near that. So I hope you enjoy my long answer
Matt

For a good explanation of working batteries in series and parallel see this link. It also gives a good explanation as to why it is better to run two batteries in parallel than two in series.

PeterD

As other says – connecting 12v batteries in parallel will double capacity and preserve voltage, while connection 2 6v ones in series double voltage and preserve capacity. From volume/weight/price ratio it is not much difference and it depends on manufacture or deal what you have from distributor. 12v parallel setup has advantage to supply more amps, thus preferable for winch applications. However it has disadvantage – if one battery crooked it eventually shorten another battery life. It is often impossible to notice that one battery playing funny until it is too late – thus regular check under load highly recommended. 6v series setup IMHO better for low to medium amps drawn and you definitely notice straight away that one playing funny, but it gets more effort to crank more amps from such setup. IMHO for fridge and caravan application 6v series setup perfect. In fact 6 2v even better.

Serg