RJ, if you have a voltmeter, you can see if the trigger is working. I think it goes from low to high (0 to 12V) when it turns on and high to low to turn off.

As for multiple triggers, you can try a splitter. Here's an example.

5-Way Multi Headphone Splitter, 3.5mm Audio Stereo Headset AUX Adapter 1/8" Earphone Earbuds Extension Cord, Compatible for iPhone/Samsung/LG/Tablets/MP3 Players - Honesun (UV Coating White) https://www.amazon.ca/dp/B07R3QL594/ref=cm_sw_r_cp_apa_i_M2OHDbFJSWB2S

One thing you ought to know is that one trigger output may not have enough current to trigger multiple devices.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated