(In reply to comment #2)
> Unfortunately I can't make both drivers work with glamor now, X fails to
> start.

The log file looks like it starts up fine; it looks like there's a VT switch
before the end of the log. Why do you say it fails to start? Is there more
information in the X server's stderr output?

(In reply to comment #4)
> If I remove xorg.conf (both cards still plugged in), I have one screen
> working (radeonsi) with correct colors.

Looks like the Evergreen card should be available via DRI_PRIME in this case.