A while back when I first got the second iMac, I controlled both iMacs from one keyboard and mouse. Nice. I used one mac for the internet surfing, and the other for working.
I did that wirelessly, but I also got the correct cable (which I never took out the box) that is mentioned in the link you sent.
I have the cable, and I have 2 macs, it is just that the 24 will not act as an external display as the 27 will.
I was just casting about for a way to harness the idle 24 to be used as a second monitor, but I guess that is not in the cards.
Thanks for all the help and expertise.