If you are trying to connect iMac A to iMac B through their video out ports, then this is not going to work.
Target mode calls for A and B to be connected via a Firewire cable; 800, 400 or 800/400.
The MiniDisplay port and the mini-DVD ports are both outbound ports and will not receive a signal.
Can I connect iMac (24"Early09) miniDisplayPort "Primary Computer" to iMac (24"Mid07) miniDVI "ExtMonitor" using TargetDisplayMode. (Found a mini-DVI to mini DisplayPort connecter with USB power and audio passthrough online.) Possible?