Second Leisure battery wiring

[westyguesty]

I have one leisure battery fitted under drives seat, and it all works fine, charges well on hookup or with engine running, very little voltage loss. It's a 65ah battery and I want to add a second battery under the passenger seat, of the same size / capacity. The one I have is only 3 months old, so could probably get one identical. Can I just run a wire from the +ve of the battery I have to the +ve of the new battery, and add an earth to the new battery.
Wouldn't is just act as a 130ah battery (more or less).
I was planning to use 3 core mains cable, but use all three cables, and do I need a fuse at both ends.

Thanks.

[lhd]

See wiki its all in there somewhere.
I think all you have to do is run a wire between the live terminals and then just add an earth to your new battery..
You will be hard pushed to fit a bigger battery than 100ah under the seat if it swivels.
I have a 100ah Elecsol battery (£97) it fits under the swivel seat comfortably but does not allow the battery flap(Westy) to shut.

[Mocki]

if you are going to twin them you really do need 100% identical batterys, same age, same condition everything, AND you need to feed the charging from a central point, so they get exactly equal, because if they get out of balance one will pull the other down, and you will end up well below capacity...... also i think you will struggle to charge two from the alternator to capacity, you need to top them up from the mains to get full effect.....