Theory is all well & good, but some of us have a budget and a hull.
Your current combo is probably not too dissimilar to a scaled up version of my first RIB- an SR4 with a 25 Hp on the transom. Read the forum here you'd think I'd have struggled to get out the marina!
needless to say I had some amazing cruises with that boat.
Your hull is definitely set up for twins, and I would guess that two of your current engine would do nicely (check the maker's plate / book of words for actual HP ratings!)
Now, assuming you have that engine already or it's a nice easy / cheap buy, as I said before, the big issue might be finding a twin for it, BUT old engines appear all the time, and I would be reasonably confident you could find it a twin at a sensible price.
Now, above there are many posts about "buy a new <whatever>, but I will throw this into the mix: I bought my 1973 vintage 60Hp clamshell for £200 with no guarantees (and the fallout from that decision is contained in many an old post on here!). A used modern 4-stroke 60Hp would have set me back the best part of £8K. Now, I agree the modern engines are probably slightly more fuel efficient (definitely quieter!), but how long will it take me to burn enough fuel to equal a £7800 purchase saving? (OK, it's more of a £6000 saving by the time I fully rebuilt it, but the point still stands!)
Me? I would Google / Ebay / whatever you local boat auction is for that style / HP of engine (remembering it's a rebadged Merc) and see what a twin would cost you. My gut feel is that would be a long term easier way to go based on sawing transoms about & the "lardy diver kitting up" comment above...
Also do a search for twin single - if you want backup a "big aux" isn't such a bad thing!