so I'm trying to plan out my future power needs here in my 272 and I'm struggling with what to do about batteries. I was hoping you all had some experience you'd care to share.
currently I have what I believe to be the original setup which is twin battery with a single perko switch. the previous owner put in two deep cycles so I'm not sure exactly if it's running on one battery to act as starter for both engine and an isolated house battery or both acting as starters with one of them pulling double duty as starter/house. before I sort all that out, do I need a 3rd battery to act as a house if I upgrade my stereo? I'm hoping to add a sub and a decent amp...but by no means am I out to compete with the flat bill crowd. doing that I'd think I'd have to configure it so that one battery is a dedicated starter and the other act as the house battery but is one starter battery enough to fire up both engines? is that "risky"? I figure I'd add an ACR to the system at the very least to avoid any incidental discharging of the starter battery. if I do add a third battery where the heck can I possibly put it...272's aren't really known for their spacious bilge. suggestions?

I have a pretty sweet system in my boat and run two 850 cca starting batteries. When we stop and crank the tunes I switch to 1 or 2 and then back to all when its time to go. I have two 10's and four 7.7 coax's run by two 600 watt digital amps. Never had any problems with the 60 amp alternator putting the juice back in which ever Bat was used.

I think you will appreciate the 3rd battery, but can get by on 2. I have a good stereo, not overkill but it sounds awesome with one sub, four 7.7s, four 6.5 (skip those and just get all 7.7s!!) and I get by all day on the 2 batteries. Right about the time my house battery is getting low is when I'm firing the generator up to put my kids to bed...so I get a good charge. If I didn't have the generator then I think 3 batteries would be a must for me.

I put an Optima sealed AGM battery in closet. Connected to amps in closet. No voltage losses with 20' #4 cables.
Worked well for me the closet is a waste of space anyway.
3rd amp was married up(sandwich-back to back mounted) when mounting amp in closet with amp under sink.
Put a computer cooling fan in a spare door for under the sink to shed heat. Wired it to amp power. So when amp was on, so was cooling fan. Removed decanter shelf behind sink. Warm air discharged into cabin.