Twin 115 (normally same block as 90hp) would really be comparable to a 200hp.
My 90 four stroke on average used no more than a litre per mile, my 150 optimax also used no more than 1 litre per mile (+ oil costs). All average and cruising speeds
Things to consider-
-you can use up to double the amount of fuel when in rough seas!
-Cost of counterrotation!
In my opinion get a Susuki 200hp v6 four stroke and 15hp aux, you will be under a litre a mile at cruising speeds. You will also save yourself about 5k.
In my opinion these days, the only reason you would go for twins is if you need more hp than 1 engine can produce.
This is why i'm going for twin 250's