You will get many opinions on this topic.
Twin outboards will give you better control and manuverability
Should one engine fail you will have a backup to get you in...good thing when far from shore.
Waht you have to consider is the weight, drag, performance of twin vs single, all boat manufacturers should be able to supply you with this.
Next is cost on 2 vs 1 engine......depending on where you go if you picked up 2 Yammi 4 Stroke 115's it is close to same cost as 1 Yammi 250 4 Stroke.
Next is maintanence 1 vs 2 engines not a big problem if you do a lot of it yourself. If you have the work done, cost is doubled.
I have had both single and double, both have plus and minus points.....
..I will say I now have a 24' Grady with a 225 Yammi, my next boat is going to be a 25' Parker with either twin 115 of 150 Yammi's
But i would consult the boat manufactuer before making any moves, hope it helps
(we did have a post on this subject quite some time ago)