Brian,

How many layers are you putting on?

If you are covering the hull or deck, I would definitely advise against wet on wet. It would be next to impossible to get the cloth nice and smooth without wrinkles. If you're doing a much smaller area, wet on wet isn't a bad idea.

For multiple layers over a large area, you can try wetting out multiple layers at the same time or wait until the previous layer isn't tacky anymore. Spreading/smoothing cloth on a tacky surface just doesn't work. Don't wait too long, however, because you'll have to sand between coats for adhesion. I use MAS and recoating within 12 hours (in hot Miami, FL) is fine, more than that and I have to sand. The rule of thumb, like JohnK said, is if you can easily make a dent with your fingernail, you can recoat without sanding. Hopefully you're using a blush free epoxy and you don't have to worry about blush.

Good luck,

Pedro Almeida