i can't give you a specific starting point but whatever you do the biggest thing i would recommend is an ATACC. it will save him HUGE amounts of stress. the only reason i tried twins on my mod sled was b/c the ATACC worked so well with the single. installed the twins, adjusted the jetting and it ran perfect all day, everyday from 0 to 45 degrees and 0 to 4000+ feet. (didn't have a chance to test it beyond that range) truly was the best thing for the money i ever put on a carbed sled, including the twins. plus if you have any questions the guys at HOltzman are awesome and have amazing customer service.
as far as your jetting, if it is running correctly now, i would bump the main jets two sizes and raise your needle at least one clip. you will probably need to bump your pilot up one, maybe two sizes. don't know those carbs (can't remember now) but depending on adjustments you might be able to add enough fuel to the pilot circuit with the fuel screw and/or air screw. (out with the fuel, in with the air) but it would be better to change pilot jets.
good luck and enjoy
pv
