Firstly, welcome to ribnet!
This is a perennial discussion round here!
In summary: Maybe! The only definite thing I can say is that you will have the same rated horsepower at the prop(s)
There's a lot to consider -
Drag through the water goes up as a cube of the speed, so , for example the difference between 2x 100Hp Vs 1x 200 is going to be greater than, say 2x25 Hp instead of a single 50. The smaller Hp will be going slower, so genetrate less drag, so less difference. Also at the bigger sizes one "lump" can be sold as sometimes 3 or 4 horespower ratings, so twin whatevers can end you up with a very lardy / draggy oversise lump of plant on your transom.
The fact that most manufacturers will use the same physical lump of engine and tune (or more usually de-tune for the smaller horsepowers), means that the single / twin argument varies between makes and models for a given overall HP. Net result if you go up the horespower range & compare an Xhp with 2 off <X/2> HP, the heavier setup will usually alternate between the single & the twins as you creep up the overall power scale. And if you do the same with different manufacturers, you'll get a different spread of lighter setups. Same goes with leg size (drag), although some legs will cover 6 or 7 different engine sizes. (e.g the leg / gearbox on my old Yam 55 was the same part number as the one on the 90 - the powerhead changed roughly every 2 sizes as you crept up the HP scale)
Then of course you have two propellors, so in theory twice as much grip, so can reduce diameter a bit (less grip per prop), which reduces the torque required to turn the prop, so you can then load them back up (to prevent the engine over- revving) by upping the pitch = more speed...... But then you're into looking at torque curves for a given powerhead........
In summary it's not a simple answer! You need to do a LOT of research, and remember that some transoms are not designed for twins......
Your example of twin 50s Vs a single 50, of course you'll go faster - you have twice the horses to push you along. Doubling your speed is unlikely, simply because as you speed up, the water drg on the engine goes up as the cube of your speed, aerodynamic drag also does, but with less of a steep curve. Because of this non constant drag, double speed for double horsepower is very unlikely.
That was a long post for a one word summary!
Hope it helped.
