To be fair v4l2 sometimes needs additional processing to allow multiple processes to use same webcam at same time. At least for those applications who use libv4l because I’ve seen mentions that this is because for some reason libv4l checks that camera is not in use.
Reading the fucking manual suggests that V4L2 is totally fine with multiple programs using same webcam without any workarounds, just only one program can set resolution and other stuff.
EDIT: found mentions of dmix in 2004. Will I find mention from 2003 to finish with round number? Also hardware mixing was in ALSA since creation, but it required hardware(thanks, cap).