I've never done one, but this local shop does a ton of them, but I think their just using two upgraded stock alt's (usually 90 amps) www.boatstereo.com
The one guy who's had alot of exp w/ HO alt's is grant @ soundillusions. Give them a call. I went the cheap route , and foundn] some 135 amp ($140/piece) alt's to have 2 burn up in a matter of months. From what I've lkearned you can take a 105 amp alt and get it to max out @ 135 amps using cheap parts and it's a crapshoot to see if it will last.To do it right basically take a 105 amp alt, gut it and have all Heavy Duty parts put it to get to a true 135 amp (but it will cost you , close to $400 ish)Bigger than 135 from what I've foudn out gets really expesive. Don't forget tha typically HO alt's will haev realtively low output at idle. And to see full output your going to need to be running in the neighborhood of 3500rpms. How often do you do that? Grant's Balmar charging system seems to be working well. you can get on his site and see pics of the install. that may be better than going with dual alts?