The normal way to work it out is.......
Fuel consumption is approx a third of the horse power at wide open throttle so a 90hp would use 30ltrs an hour. (2 stroke that is)
4 strokes use about a third less again so.....
a four stroke would use about 20ltrs an hour at wide open throttle.
So twin 90s will use approximatly 40ltrs an hour and a 200 will use about 44 ltrs an hour. But its not that simple as your running twin 90s you have more drag as you have two gearboxes that brings the consumption down a bit so your probably going to get about the same consumption. Althought the single 200 will be quicker than twin 90s by a fair bit. but you do have reliability with two engines.
These figures are for Mariner/ Mercury engines not sure about other engines.