several years ago I made a prototype to match a particular request.
The boat was a Tomcat 5metre catamaran.
I installed a Mercury 60HP outboard, fitted with a jet leg instead of the standard prop leg.
Due to the fact that the speed was lifting the hull, I had to make some special GRP extension, but it worked well.
The speed was approximately 23 kn, normal when you know 30% of HP is lost.
The result would have been much better with a twin engine indeed, because in this case the waterflow entering the turbine would not be affected by the hull heigth, turns, etc... wich make it behave like cavitation.
This is a cheap and very efficient solution : buy a pair of standard outboard engines, and a pair of waterjet extension.
The extensions are very cheap - approx 1500 to 2000$ - easy to install, and they exist for most engines up to 200HP and up. Much less than waterjets packages, and you have both possibilities for the same price, waterjet or prop either.
I will fit my next rib (Tomcat 570 with 2x50HP, or 610 with 2x70) with that.