Check the coil.... Had a 28' Pursuit with twin Merc IOs. Had to carry spare coils all the time. Those Mercs would burn them out. I'll look to see if I still have one in the shop, even though that boat has been gone for ten years.
you have to goose the gas a little to excite the alternator first.
Some will self excite. Some just wont.
I have twins. one will-- one will not.
I have to goose it a little to get it going then it's fine