Without knowing more about the internal design of both equipment: no, I wouldn't recommend it.
One: the exact way they are wired is unknown. The monitor and dock might be common-ground (through earth and cable shield connections), or low apparent resistance anyway, but that doesn't say whether there are current-sense resistors for example inbetween. Creating a DC ground loop would disrupt operation of such a function. In particular, you can't avoid DC current flowing on parallel paths, including the data cable itself; this creates voltage drop, which may violate data receiver voltage ranges, corrupting video, or causing it to malfunction or fail outright.
Two: fusing. You didn't mention fusing the common supply; perhaps this was assumed a given, but I mention it for completeness.
Note that neither device expects the transient / inrush / fault current available from a much larger supply (let alone a type different from what they were designed with), so there may be operational or indeed destructive possibilities here, even with fusing in place (because any one device can hog 100% of the supply's capacity, which is now, say, more than double what each one is designed for?).
Three: voltages. I put this as a lower priority, because it's either something easily enough solved with fudging things around (there likely is compatible overlap as you anticipate), or by using one or two regulators to make the necessary adjustment (including more hacky solutions like series diodes).
A common solution is very likely, I agree, but to verify it as a design matter, requires far more knowledge than you possess (read: reverse-engineering much of the power input sections and grounding arrangements in both units).
Note you still must provide a safety ground, to whatever extent they had one in the first place. The dock might not; the monitor most likely so.
From the monitor User Manual:
The AC plug isolates this equipment from the AC supply.
The Class I pluggable equipment Type A must be connected to protective earth.
For models with adapter: Use only the power adapter supplied with your LCD Monitor. Use of another type of power
adapter will result in malfunction and/or danger.
They never actually specify what type of product each model is, as this manual is generic... but I would assume the AC power cord for the GW2480 is a standard 3-pin type and the equipment is therefore "Type A".
Curiously, I don't see mention of power supply compatibility in the OWC Thunderbolt 3 Dock User Manual, but it would be safer to assume it should use its own power supply.
As for the easiest solution that's still in the spirit of the original equipment: you could have a common rail (which needn't be 20V, it could be anything; and at these power levels, a somewhat higher supply wouldn't be a bad idea), with isolated converters for each device, thus replicating the isolation that each original power supply offers. This is no small feat, as isolated supplies are far more liable to generate EMI than simple buck/boost converters (common ground) are, and the filtering is more complicated to solve. But this is adding more converters, and at that point, who's to say the "common rail" can't be 160 or 320 or 400V DC, or indeed 120/240V AC -- as the original supplies themselves use (internally and externally)?
Supplying AC would be a downside (adds another converter -- an inverter, that is), but it is the easiest ready-to-go and approved solution here.
Also just in case -- no, modifying the original power supplies to accept wildly different inputs (like 20V or 48V or whatever) isn't really feasible. It is possible that the supplies as-is can accept DC input, but this may still be unsafe or unwise for various reasons (fusing, distribution/switching, warranty), and you probably still need a converter in that case (but a high voltage boost converter is at least more efficient than a full AC inverter).
Even then, mind that the monitor (and the dock by presumption) won't be warrantied under these circumstances. Even if the design parameters match perfectly, they're simply not the original supplies, period.