I have two 27" iMac connected via a Thunderbolt cable. I press Command+F2, the target computer joyfully becomes a second screen for the primary iMac. Joy of joys!
I come back in the morning, or after lunch, and I press a key on my computer. The display lights up, everything is where it should be.
Unfortunately, the display computer stays off. I haven't found a way to bring it back on, other than pressing the power button, waiting for it a little (this disables target mode), then pressing Command+F2 again. This, of course, re-enables target mode.
Any idea what I could be doing differently and better, for a wholesome dual-27" experience?