Dont know the Gemini WR600 but if you want some speed 2 x 50HP sounds a bit underpowered to me.
Twins are not as efficient as a single and I would have made a guess that a 6M rib should run upto about 150HP.
You need to decide between fuel consumption/speed of a single or the redundency/safety of a twin setup if you have a limited budget (who does'nt). You cant have both.
As far as 4/2 strokes, in the engine size bracket 50-75HP I dont think their are that many HPDI/Optimax engines. Going for a single 100HP-150HP gives you much more choice in the hitech 2 stroke market, and the good fuel consumption that goes with them.
One thing to watch for though is although 4 strokes give you good fuel consuption they are often heavier and twin 4 strokes maybe very stern heavy.
Maybe others who run outboards can help as im a diesel convert
, but I dont think I will be far out with the above.