Ordered a 1/4 TRS to RCA cable to test the FS-7 as a Reverb / Tremolo footswitch, in similar fashion as OP.
Hosa TRS-204 1/4" TRS to Dual RCA Insert Cable
Since the Z-LUX operation of Reverb is default On and Tremolo is default Off....
The FS-6 Dual Footswitch has an advantage over the FS-7, where there are separate Mode & Polarity switches for each footswitch.
The FS-7 Footswitch has a single common Mode & Polarity switch.
Test proved that the Reverb switch Polarity had to be Normally Closed, so that when the assigned footswitch (A) is activated the LED would emit.
Also, the Termolo switch Polarity had to be Normally Open, so that when the assigned footswitch (B) is activated the LED would emit.
So, one LED represents activated and the other deactivated, depending on the FS-7 Polarity setting.
Additionally, a mild switch noise is heard when actuated. (not a pop)
FS-6 is 9V internal battery powered, where as the FS-7 requires 9VDC external power.