Only one side at a time works (v4.1)

[Arne-Berner]

Hejhej! I've been tinkering a lot with the crkbd now and I've finally got both sides working. the only problem is, that they only work via usb-c.

Problem:

As soon as I try to connect them with trrs only one halve is powered.

What I would expect to happen:

I would expect the left and right halve to work when I flash the left keyboard halve while both sides are connected.

What I've done:

The cable works with an older corne, I checked continuity and everything seems to be fine.
When I power the left have (the LEDs and switches work there, when plugged in) I got power on the right halve, but the LEDs are not on. (measuring the power line gives me 5V/3.3V depending where I measure)
I flashed each one separately for testing purposes. Could this have been the issue?
I'm using qmk flash with the following command to flash them

qmk flash -kb crkbd/rev4_1/standard -km default

Is there any line I can measure with a multimeter/(cheap) oscilloscope that could help me here? Do I need to change any default configs in qmk?