Found the source. This has been discussed on multihulls mailing list a few times. One message By Ken Harland from 2006 that I found easily:

"I undertook a similar configuration in a 32' cat also using two jet ski motors.  I was drawn to them for their compactness and relatively light weight for the horsepower. 

Unfortunately, I had tunnel-vision when it came to getting rid of the loud obnoxious two stroke hanging off my rear beam that I didn't research the project adequately. 

Jet drives real advantage come primarily when used on a planning hull design.  Although it can be argued that my 3000lb boat can and does plan, it won't under motor power, even with 70HP total!  I could never go faster than the old 9.9 used to push it!

And after multiple iterations, I never got a decent reverse system worked out.  I should have spoken with the local hot shots that drive 455 jet boats...they can't even back off the beach when lightly driven up on the sand - no matter how much horsepower!

I'm now using an 18HP 4 stroke Nissan that while it has nice power, unfortunately vibrates like you wouldn't believe!"
 

So he says that the total power of 70 hp with two jet ski motors resulted less speed than 9,9 hp two stroke outboard. The efficiency of propulsion system has really bog impact on the outcome of the whole system. Good efficient propeller is the way to go.

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Arto