Actually, looking at ur pics again, the transom is already cut out as wide as it can, but twins would still be a bit tight.
Having the performance, and redundance of twins is certainly a benefit, especially for any offshore use, my last 2 offshore rigs had twins, and I don't think I would go back to single at this stage of the game...I limped home quite a few times when things went wrong.
Aside from cost, the biggest con would be rebuilding back the solid transom, and making it look good where the cap wraps the stern.
The AG platform bracket looks like it would do the job, and give u the swim platform to boot, Armstrong also makes brackets to check out.
http://www.armstrongnautical.com/brackets.htm
Lets do it man lol