Q: Wrong Target Display iMac

I am using a Mid-2010 27 inch iMac as my main machine and I also have a 21.5 inch 2009 Late-2009 iMac. I'd like to use the 27 inch as my main computer with the 21 inch as the secondary display, so I purchased a double male ended Mini Display cable, whenever I plug them in, the 27 inch iMac becomes the secondary display to the 21 inch, I hit Command-F2 on the 27 and it goes back to normal, but then I hit Command-F2 on the 21 and nothing happens. It seems as if I'm only able to make my 27 iMac an additional screen for my 21 inch, but not the other way around. Any ideas? Thanks in advance!