Fly by wire does make it easier to do that sort of thing. I have seen (tho' can't remember if it was in a mag or on t'net) a U-channel type thing that dropped over the top of the levers to work many as one. It was long ago enough that PT was a very expensive option, and "anti knock into gear accidentally" catches hadn't been invented so probably explains how they got round that! Also throttle cables can be run in paralell (e.g fly + inside bridges) so I'm sure it's not beyond the wit of man to allow independant control at low RPM & have a "pickup" on one lever that grabs another engine's cable on the way past.
Back to Tom's original question, most OB side mount throttles are designed so that the "T" of the top of the lever will line up nicely if two are mounted back to back, so even small handed people can grab the two together. Dropping into gear should only really be done individually in that kind of setup anyway. and is nigh on impossible to do two at once with the catches. Twin setups are not uncommon on Boston Whalers, thay have a forum all of their own, I'm sure a trawl of that will give plenty of pics to see examples.
Synching trim on older engines can be as simple as knowing the boat and listening out for that "sweet spot" in the noise as you trim one up & down to match the other.
..and we all have dream boats! - There's guys here started with diddy sibs & worked up from there. I have owned a few preowned & in some cases unloved (No offence Al, yours was one of the better ones!) boats in ever increasing size, but as taking engines to bits is one of my other(!) hobbies I can live with "low price failures". For what it's worth, my dream rib isn't too dissimilar to yours......