I have a 375 AH house bank plus a starter
battery, connected via an ACR. A Delco CS130alternator was on the boat, and after researching it's suitability for the purpose, I had a shop convert it to external regulation for around $60. I got a Balmar MC614 regulator and the combination has been fine after 2 seasons. I was concerned about keeping the 3/8" belt and looked into getting doubles or a 1/2 inch. But using the belt load manager on the VR, and just notching it one step down, I've really noticed no appreciable belt wear.
The one problem is that the single foot of the Delco needed machining down to align the pulleys, and they still are not in great alignment.
But as has been pointed out, unless you really run the batteries down, you're not going to be putting a heavy load on the belt for any length of time, and you have the small engine switch if you need it. You can actually observe it if you have a battery monitor--and I think it makes good sense to have one.
Another problem I had with switching to dual belts is that you also have the water pump
pulleys to consider. So I don't think I'm going to need to do anything with the belt for now.