short term battery fix

ok so i only have one battery in my boat right now and i just bought a second one for the fourth of july. i am planning on running them with a perko switch and keep them seperate but i have a problem. the switch wont be here before the fourth and i really dont want to run my stereo with one battery again. so my question is....could i go ahead and hook the batteries up together to give me more amp hours and not drain my boat dead? cause i know if i go with only the one its gonna die. and what would be the best way to tie them together parallel or series?

If they're the same battery and the old one is in good condition then you can parallel them together. Series would double the volts to 24. I ran mine like that for about half a season and it was all good.