Gas/Electric powered twin OK Prowler catamaran.

This is one way for a guy to get into power boating pretty cheap, yet build it his way. 

Crusty, but trusty! 

From an engineering standpoint, it's a great idea. But, it's like a $4,000-$5,000 setup if you were to buy everything brand new, probably 3k for the 2 kayaks alone. But, if I had 2 kayaks already and wanted a platform setup for having more than 1 person, this would be close to what I'd build.

That looks like it would be a tank or more rightly a turtle, probably cheaper just to get a pontoon boat.  Or he could get one of these.

^ That has a 2hp engine, the mokai has 6hp honda.

If you add even an electric trolling motor don't you have to register the yak like a regular boat?

Exempt vessels

-Canoes, kayaks, or vessels that don’t have a motor or sail (strictly human powered).
-Vessels less than 16 feet long with a motor of 10 horsepower or less, if used on non-federal waters only.

I take it non-federal waters is limited to private ponds/lakes?

Also walmart has SOT kayaks for $449 for cheap floats and a twin-wall white water one for $249. If it just needs to float...