GEIA KAI XARA MARIE
I use to have a 5 mtr GEMINI with a 75bhp Marine 2stroke and 17" pitch aluminium Quick Silver standard Prop. Very good boat and I am very sorry that I have sold it.
Now back to your question. If you listen to all the prons and cons about 4-stroke engines (and I have done my sums so far) for a 115 Yamaha 4-stroke you will need approximately to run it for 8000 miles in order to achieve a break even point (low consumption vs purchacing costs). Plus the fact that all of them are car engines, complicated mechanisms, with computers on them and VERY EXPENSSIVE repair bills. Also they have not been tested extensively in the sea environment and they are heavier than the 2-stroke equivalent.
On the other hand two stroke outboards are cheaper, lighter, simpler, have been tested extensively at sea and currently they all run (more or less) without any problems.
I do not understand why you want to mount a twin installation. It will make your boat heavier at the back and the behaviour in big seas will change dramatically (so I was told) plus the fact that I do not think you can find left and right screw engines under 100 bhp (correct me if I'm wrong).
In my humble opinion may I suggest that you go for a YAMAHA 2 STROKE rather than the 4-stroke??
Is just a thought and my personal view on the matter. Others of course may argue differently.
KALO KALOKAIRI STHN ELEFTHERH KYPRO!!