I just wired up things the way you described, but I need a little more detail to be sure I've replicated your setup. In the end, it probably doesn't matter.
I used two S1s at my gig last weekend and ran in stereo through the app. I played music from my iPhone, which was connected via Bose Connect. I set the bluetooth speaker volumes the same and their volume was equal.
I did this too.
I ran two mics and an acoustic guitar through my T1 mixer, then trs from the T1 to the first S1
What channel did you connect the T1 to on the first S1 Pro system?
and then TRS from the output of the first S1 to the second.
What input did you connect to on the second S1 Pro?
Let's say you've got the Bluetooth connection going with equal volume on both S1 Pro systems and you have the Channel 3 volumes both at 12:00 o'clock.
With or without anything connected to S1 Pro channel 1, when you connect S1 Pro #1 Line out to S1 Pro #2 channel 1, when you turn up that channel to 12:00, the volume of the Bluetooth signal will get much louder.
On S1 Pro #1, the Line Out includes the Bluetooth signal on Channel 3.
When you connect S1 Pro #1 to S1 Pro #2, you are sending the Bluetooth signal from #1 to #2 through the wired connection.
The result is: You have added the Bluetooth signal from #1 to #2, and the result will be - the Bluetooth volume on #2 will be louder.
If you are going to use a wired connection between the S1 Pro systems, send the mono Bluetooth signal only to S1 Pro #1.
HOWEVER, when I turned the channel 1 volume up on the first S1, the volume of bluetooth music on the second S1 got much louder!
I can't replicate this - but if I turn up the volume on S1 Pro #2, then the Bluetooth signal gets disproportionately loud.
I can't remember if I tried this with nothing plugged into channel 1, but I think I did and the issue was the same.
This would happen if you turned up the first S1 Pro channel 3.
This seems like a bug to me, or poor design. If I'm playing in stereo through bluetooth, the mixer channel should not affect the Bluetooth volume.
Is it user error? If so, what?
The S1 Pro systems are engineered to work this way.
A wired connection between the two S1 Pro systems sends the full mix of the first S1 Pro (Channels 1, 2, and 3) to the second S1 Pro. If you are running a Bluetooth signal in the first S1 Pro, that Bluetooth signal will get sent to the second S1 Pro.
Typically, the reason to connect something to the S1 Pro Line Out is to send the full mix to a second system. That's what's happening here. That works great when you are sending the output of an S1 Pro to a house system, or to a second S1 Pro to extend your coverage.
I understand you might want to preserve the stereo image in the Bluetooth, but if you're connecting them together to send the T1 output to both in dual mono. The stereo image is going to be obscured anyway.