I popped a pair of the Tear Into jet drives into a plastic Atlantic Yacht, which is probably twice the size of the Tear Into hull ?

The build details and a video of it running are in this topic somewhere - the result was a quick take off, it went quite fast but I can say for sure twin drives doesn't give you twin 'power' as sometimes a fault would occur so only running on one drive, yes it was slower but not that much slower.

The only advantage I can think of for putting twin drives into a Tear Into would be the motors would be more efficient (run slower), they wouldn't have to scream their nuts off to get the same performance as a single jetdrive.