One sub in two systems: Is this possible?

The Storm III has both the high level speaker inputs and low level RCA inputs. So yes, with that sub you could have it wired to both systems, and both inputs have separate volume dials for each input. You will just have to fiddle around to see which input sounds best for each system (REL usually recommends the speaker level, so that will likely be the best sounding).

If you put an active crossover (NHT X-2) and/or a x-over/subwoofer manager (Velodyne SMS-1) ahead of the sub, you might be in good shape. IIRC, both of these units have SE and XLR inputs/outputs. You could run each set to a different system (adapters may be required for XLRs in your case). I believe that each of these would default to the active signal - you might check with the manufacturers to be certain.

One additional nice touch with the Velodyne - it stores multiple presets. You could optimize the level, phase, slope, etc for one system and save it to preset #1, then optimize the settings for the other system and save as preset #2. Just toggle for the system in use.

Good Luck.

Marty