Switching "primary" imac in Target Display Mode I work in an office with a bunch of old iMacs sitting around. I thought I'd get clever and strap a few together to make a couple extra workstations. The goal has been to pair a 27" mid 2010 with a 21.5" mid 2010, using the 27" as the primary machine since it has much better specs. Both machines have mini DisplayPort, rather than thunderbolt, and are connected with mini DisplayPort cables.

The problem I'm running into is in pairing the machines, the 21.5" imac becomes primary machine and I can't seem to reverse this using Target Display Mode. I can toggle between the machines running separately, or the smaller one acting as the primary.

Is there something I'm missing? How do I switch them?
The FAQ for target display mode is pretty clear on the requirements. 

You get to bring in external video if it matches exactly the requirements. Display Port isn't the same as Thunderbolt, so perhaps that's the issue. You also need the master iMac to be able to drive all connected displays.

If you can't use the hardware directly, consider software to merge the screens such as AirDisplay or Screen Recycler.