Set xrandr to share crtc

[Vojtěch Aschenbrenner]

Hello list!

Do you know, how to force xrandr to share crtc between more outputs? I
have only two crtc (0 and 1) and 3 identical displays. Two of them are
connected via DP and one via VGA. Theoretically, it should be possible
to enable all three displays by sharing crtc between these two DP.

Configuration:
Lenovo Thinkpad x220 (Sandy Bridge) + Ultradock which has 2xDP, 1xVGA.

I think that the key is to tell xrandr to set the Clones value to share
one CRTC but cannot figure out how.