Hello gshukill,
I'm not sure how your main iMac is going to sleep but the behavior of the external display iMac depends on how the main iMac is sleeping.
OS X Yosemite: Use another Mac as a display
Learn what happens when the primary Mac goes to sleep and wakes up
The external display Mac behaves differently depending on how the primary Mac goes to sleep.
- If you put the primary Mac to sleep: After you put your Mac to sleep, the external display Mac goes dark for a few seconds and then displays the windows for its own apps. If the external display Mac does not display the windows for its own apps, press Command (⌘)-F2 on its keyboard.
- If the primary Mac goes to sleep because it was idle: The external display Mac goes dark and stays dark. If you want the external display Mac to display the windows for its own apps, press Command (⌘)-F2 on its keyboard.
If the external display Mac doesn’t show the right desktop after it wakes up: When the primary Mac wakes up, the external display Mac displays the windows for the apps on the primary Mac. If it does not, press Command (⌘)-F2 on the keyboard of the external display Mac.
There's another article that states that sleeping either computer exits TDM.
Target Display Mode: Frequently Asked Questions (FAQ) - Apple Support
How do I exit TDM?
To leave TDM, press Command-F2 on the keyboard of the iMac that is in TDM. You can also exit TDM if you shutdown or sleep either computer or detach the cable.
That would explain the need to re-enable TDM when the computer wakes up.
If you aren't sleeping them for long maybe a screensaver would be a workaround.
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Nubz
