My own thoughts, from the "single vs. twin" thread and from the "twin diesels on one screw" thread:
Fewer engines should be less expensive than more.
A single often gives better access for maintenance and service. (At least ours did.)
I'd like to go back to a single-engine boat. (Not likely to happened anytime soon, but I can dream).
Having said that, since we're inshore with towboats everywhere, I'd likely be happy enough with that, as is.
Having said THAT, if I were going offshore more often, longer distances from the towboats... a get-home (or at least get-within-tow-range) sounds prudent.
An option that uses the same shaft/prop as the main seems OK at first, but there's also risk that "the problem" (that may arise) isn't the engine at all, but rather the gear, the shaft, the prop... Still, a "small" diesel ought to be much less expensive than a larger "twin" would be. Could potentially be located so as to not lose all the improved access space?
An option that includes a whole 'nother gear/shaft/prop seems OK at first, although that also means exercising that system periodically. (Not difficult.) And the time and expense of yet another system to maintain.
Using the genset diesel seems OK at first, but then there's the potential problem of underloading under normal circumstances, or else the genset diesel might not be large enough to be particularly useful. (Might be some balancing act that's possible, there.)
The option of two different size gensets is intriguing. I assume that means fixed-rate RPMs (e.g., 1800) for both, even when the larger is being used as a get-home? In any case, I'd guess the larger could drive either the main shaft, or a separate shaft, depending on the specific installation...
I'm of three minds.
But doubt I'll have to "solve" this one any time soon for our own purposes. Just examining the ideas...