Although this is a perfectly reasonable solution, Unfortunately this particular design happens to be used in off-grid/solar sites on a fairly regular basis where every watt counts. I am going to investigate this as an option though, since doing something similar might make some sense... i.e. "pre-regulate" at the highest voltages, even if it's a simple 2:1 DC-DC converter.
Holy hell, how many of these things are going to be deployed? Ten thousand on a site? You'd think they could muster some cleaner power if there's that substantial of an installation going in!
If this is one-offs, bwahahahah, no,
you literally waste more in as many breaths talking about it as saving that one piddly watt.

I don't care if
they think everything has to save a flea-fart, reality is
their problem, not mine!
Or to put it in pure economic terms: you can spend another $10k tuning out another 10 percentage points of efficiency, or you can save another buck (or whatever) on the sale price, and potentially sell more to more customers as a result.
This is very much in the class of problems where it's more economical to spend that budget difference on marketing, rather than engineering. (And I say this as an engineer!

)
Don't get me wrong, I'm all for efficiency. Everything must be done within reason, and demanding that already such a heroic device must be perfect, is outside of that.

I looked at several controller designs, I think the problem was that I was looking for isolated designs which turn into a rather large circuit, and are fairly few between. I'm going to go back and look some more.
Another option is two FWBs, one for each input with respect to ground, so each one delivers max 60V (or whatever). Independent converters or isolators then drive a common rail, or, something (maybe some of that changeover logic, signaled by optoisolators?).
A basic logic function here would simply be using two independent error amps, in parallel. Normally this would be undesirable: you're effectively wiring two regulated power supplies in parallel, which inevitably will not share current. But you don't need to
share here, you just need one or the other (or both, if you get lucky) to supply enough power. So, the setpoints could be staggered, so when the output voltage is low, both converters run at full throttle; at the low setpoint, the first converter throttles down, then at the high setpoint, the second one throttles down as well. If the higher-set supply is unavailable, the output voltage falls slightly, but still well within nominal range.
Compensation might be awkward, because a transient change in the output voltage (period) will tend to activate both error amps; this may not be a bad thing, because recovery will be faster in that case. Anyway, the transconductance can only double (with two equal error amps acting in parallel), which is easily manageable in a control loop design. Transient response would be a little weak when just one supply is operating, but I think it's completely manageable.
Anyway, the split supply design allows half input voltage, which is helpful.
This doesn't help necessarily with a floating-ground situation, since the effective (DC) input resistance of a converter is negative. It will latch to one side or the other. But the converter sitting at ~0V will run out of power and turn off, stop carrying current, go open circuit, then charge up and go again, oscillating.
Probably it would be fine to set the "high side" controller to sense the input voltage midpoint, so it can deliver power (even if not commanded to by the voltage error amp?) and stabilize the input voltages. The increased output current would act to throttle down the one active channel (which would then have to be the "low side" controller), so they naturally share load, it's not a runaway situation.
Yeah, in fact the high side doesn't need an output voltage error amp at all, it just follows along with what it sees at the input. That'd be fine.
Yeah, that's not a bad method.

A bit complicated to describe, easier to draw out in block-diagram form, and possible to prove good behavior in block-diagram form as well.
Or it can be combined with the linear pre-regulator method, or instead of a load balancing control, just put in a shunt regulator there, effectively to bypass the one inactive converter while the other draws load current through both supplies. That would work too.
Tim