I suspect to get enough stiffness in your adaptor plate to hold 2 engines you are either going to end up with a monstrously heavy lump of metal, or pay as much as just buying a couple of Mariner Units for some titanium fabrication. - and with the current exchange rate probably cheaper to find a couple of secondhand PT units on Fleabay! - I managed to pick up an (almost) working PT unit for my old Yam for £130. One set of replacement brush springs in the motor , a replacement sealing O- ring for the motor cover and a top up of hydraulic juice later, it's a runner.
Another hint is that if you look at ishopmarine, trawl up & down the Microfiches there - the high level "look see" would be to get a large sheet of paper & chase the PT units both up & down the range (e.g from 30 to 90 HP - paper to write the part numbers down), then you can find your equivalent engine on the Merc / Yam 'fiches (depending which respray you have) & do the same there. - You'd be surprised how many of the smaller engines carry "overkill" PT units - Mine came off a 50 - also used on the 90!
Also you may find by chasing the part numbers & cross referencing known engines that both Yam and Merc parts could fit, even tho' your engine is only going to be sourced form one of them......
Suddenly the number of units you could use mushrooms out of all proportion - especially if you don't mind a change of paintwork under your swivel brackets. It does however mean you need to spend ages on t'internet cross referencing part numbers
To give you a head start, the Yam 3 -ram design goes on pretty much anything post 1979 between 50- 90Hp, and odd 40s as well.