Answer found. See 'edit' below.
I've been using Simhub with my bass shakers for ~1 year with a UShake6 seat pad. Simhub version is 8.2.3. I hooked up a second UShake6 system (not using seat pad, only the transducers). I renamed them in Windows for ease in tracking and replicated all settings (in windows) and everything works in windows sound tests for both systems. (Each system has the same integrated sound card/amplifier for 6 channels). Only 1 of the 2 systems works in Simhub. (Strange that the newer works but the one I've been using doesn't). I repeat, for the system not working in Simhub, windows settings 5.1 recognizes and tests good. Also, SimRacingStudio software confirms that the system works fine. (SRS app is not pleasing or nearly tunable like Simhub). Simhub shows that the signal is being sent on the 'sound ouput' screen (on custom channel map) but I get no transducer response. I've probably left out some important detail but most questions can be negated by the fact that the 2 systems function outside of Simhub. I've even tested one at a time in Simhub by unplugging each USB and restarting Simhub. Same result, one works while the other not. I've also reinstalled Simhub to no avail.
EDIT: Happy to say that I found the answer to the problem. For whatever reason, when I went to "settings" in left column and found that "simhub sounds action volume" was at zero. (I know that it wasn't previously set to zero because my RomainRob sounds plugin wasn't working and had been working). As soon as I set it to desired value, my 2nd sound card/amp started working (along with the other that was already working).
Saw your post and wondered how well two Shaker systems were working simultaneously under SimHub. I have a Buttkicker, a complete set of Simagic Pedal Haptics and just added the U-Shake6. The Simagic pedals appear under a different tab and are easily controlled separately, but both the Buttkicker and U-Shake6 appear under the Shaker tab. I'd like to assign some effects to one and some effects to the other, but can't figure out how. Since you have two similar systems I thought you might know if this is possible. If so, how?
Also, for your U-Shake6 systems did you do any output tuning where you set high and low pass filters? If so, what did you select?
Thanks.