I think you may be trying to solve a problem you don't have. There are arguments for and against twins, so here's some random thoughts:
I don't think a 60 or 75 will plane the boat by itself, and you will be addding additional weight at the transom, and probbaly burn more fuel. You will also almost double maintennace costs, as you maintain two large engines.
Another consideration is cost. If you were buying the boat new, and decided to rig it with twin 75's, you would spent around $8000 more than you would dor a single 115. To do it now will cost somewhere around $18,000, minus whatever your existing motor may be worth. Part of the cost will be completely derigging and rerigging the boat, adding gauges, etc., etc.
The main advantage for twins I hear expressed is redundacy and safety if "one should go down". With the reliability of today's engines, the chances of an engine just "going down" are fairly rare for a well cared for engine.
Keep in mind there are many working boats going out on a single engine- these are guys that make their living on the water, and down time means lost revenue. Yet the simplicty of a single powerplant still appeals to them.
What can take an engine "down"?
1. Bad Fuel: Can just as easily affect both engines.
2. High Speed Object Strike: Didn't see that deadhead in time? Can just as easily take out both props or lower units.
3. Major Electrical Failure: Again- probably affects both engines.
Other than bad fuel, a kicker added to your existing set up can be a better way to go. With a steering rod attached it can be steered with the helm, getting one with a manual start (even if it has electric start) means that an electrical issue won't leave you dead in the water. And in case of a high speed strike, the kicker is out of the water. Lower it down and get back under way.
It may not be as fast as the main, but it will allow you to be under way. Besides, it takes a fair amount of horsepower to get above hull speed (about 5-6 knot in the 23). A single 75 may be able to get above that speed, but probably not by alot.
So you end up paying additional money in operating expenses for the time you use the boat, on the small chance of having a failure. We prefer the kicker option.
Just my 2 cents worth.