Running two v4l2loopback devices with their individual properties - v4l2

Question

Working with v4l2loopback devices I can run these two virtual devices:
a) running the preview image from a Canon DSLR via USB through v4l2loopback into OBS:
modprobe v4l2loopback
gphoto2 --stdout --capture-movie | gst-launch-1.0 fdsrc fd=0 ! decodebin name=dec ! queue ! videoconvert ! tee ! v4l2sink device=/dev/video0
Found here, and it works.
b) Streaming the output of OBS into a browser based conferencing system, like this:
modprobe v4l2loopback devices=1 video_nr=10 card_label="OBS Cam" exclusive_caps=1
Found here, this also works.
However, I need to run both a) and b) at the same time, which isn't working as expected. They are interfering, it seems they are using the same buffer the video flips back and forth between the two producers.
What I learned and tried:
A kernel module can only be loaded once. The v4l2loopback module can be unloaded using the command modprobe -r v4l2loopback. I don't know if loading it a second time will be ignored or unload the previous one.
I've tried to load the module with devices=2 as an option as well as different video devices, but I can't find the right syntax.

Answer 1

As there is an already accepted answer, I assume your problem has been solved. Yet, I was quite newbie and couldn't set the syntax even after the answer above (i.e. how to set video2)
After a bit of more search, I found the website that explains how to add multiple devices with an example.
modprobe v4l2loopback video_nr=3,4,7 card_label="device number 3","the number four","the last one"
Will create 3 devices with the card names passed as the second parameter:
/dev/video3-> device number 3
/dev/video4 -> the number four
/dev/video7-> the last one
When I was trying to use my Nikon camera as a webcam and OBS as a virtual camera for streaming, to have full control of naming my video devices was important. I hope this answer will help some others, as well.

Answer 2

from your description ("the video flips back and forth between the two producers") it seems that both producers are writing to the same video-device.
to fix this, you need to do two things:
create 2 video-devices
tell each producer to use their own video device
creating multiple video-devices
as documented this can be accomplished by specifying devices=2 when loading the module.
taking your invocation of modprobe, this would mean:
modprobe v4l2loopback devices=2 video_nr=10 card_label="OBS Cam" exclusive_caps=1
this will create two new devices, the first one will be /dev/video10 (since you specified video_nr), the 2nd one will take the first free video-device.
on my system (that has a webcam, which occupies both /dev/video and /dev/video1) this is /dev/video2
telling each producer to use their own device
well, tell one producer to use /dev/video10 and the other to use /dev/video2 (or whatever video-devices you got)
e.g.
gphoto2 --stdout --capture-movie | gst-launch-1.0 \
fdsrc fd=0 \
! decodebin name=dec \
! queue \
! videoconvert \
! tee \
! v4l2sink device=/dev/video10
and configure obs to use /dev/video2.
or the other way round.
just don't use the same video-device for both producers.
(also make sure that your consumers use the correct video-device)