FWIW. I have a built-in gen, but I ran across a web site that has a "how to install a larger fuel tank" for a one portable generator. Maybe for two gens it would also work, with a hose "Y", and hose and cap mods for the second tank. Seems to be pretty much the same as g bears, except with the use of marine fittings.
It does have pics also.
http://www.ehow.com/how_2323144_add-...generator.html