How do I connect one iMac to to another iMac

You can't do that.

Check out this thread, though it is unclear if you can use both displays in parallel.

http://support.apple.com/kb/HT3924

Any iMac ever made does not have the ability to do this, your iMac LCD can only be addressed by the iMac that it is part of. The only way to have 2 displays on an iMac is to buy a Display and connect it to the display / thunderbolt port...

Any iMac ever made does not have the ability to do this

Apparently not quite correct:

Using a 27-inch iMac (Late 2009 or Mid 2010) as an external display

beccarock8 I don't know if it will work but if you connect your old machine to your new one and use a mini-displayport cable and launch your old machine in target display mode as directed by that apple doc I linked to ti may work...or not. The wild card here is the thunderbolt to mini displayport. Certainly the TB connection can be used to connect to a mini-displayport display. The question is does the old computer look like a display to the new machine?

Note I said use the older machine in target display mode. The reverse definitely won't work since a TB device cannot be connected to a mini-displayport source.

Confusing Mini DisplayPort with Thunderbolt