Dual pressure switch / dual pressure tank issues
As far as I know, "radon bubblers" operate at no pressure (so you can bubble air through the water to remove the radon) and therefore the well side of this system needs only to provide adequate pressure to put water into the radon bubbler. 20/40 might do if it does not have a higher input pressure requirement. Since the pressure there is zero, there's no relationship between the incoming water pressure and the outgoing water pressure.

All the house pressure and flow comes from the pump &amp; tank going out of the radon bubbler system (at least when it's operational) and that would be the one you'd want to be bigger, unless it was a variable speed / "constant pressure" pump - if set to 40/60, probably not.

If the well pressure goes to zero, it's likely that the well pressure tank is improperly set (too high) for the well pressure switch, not anything to do with the smaller tank, but perhaps that contributed to the radon system's pump overheating (if the well-water input was intermittent.) If the switch is actually coming on at 30 PSI, the tank pressure (with the system off and drained) should be about 27 PSI. The small tank should be set at 37 PSI if its pump comes on at 40 PSI. If you changed the well to 20/40 you'd change the air on the well system to 17.

Then again, running 40/60 into a small tank, that pump was probably cycling on/off a lot, and a bigger tank there (or an additional. bigger tank there, if the small one is in good shape) might well help the pump survive longer. Presumably consistent water input by correcting the well pressure tank setting might also help. I'd also correct the dubious plumbing job that left you with no pressure gauge on the 40/60 system.