Remove the line:
WSGIDaemonProcess mydomain user=myuser group=mygroup processes=1 threads=1
from the VirtualHost for 443. The WSGIProcessGroup for mydomain in that VirtualHost is able to reach across to the WSGIDaemonProcess definition in 80.
In other words, as the error message tries to suggest, the name for the WSGIDaemonProcess, ie., ‘mydomain’, must be unique for the whole Apache server.
Referring across VirtualHosts as indicated means both HTTP and HTTPS variants of site will still run in same daemon process group/interpreter.