It can be done, but I'm afraid we'll need more information in order to help. You say you're already dual-booting Win 7 and OS X? and want to put Snow Leopard on a second HDD? or is SL already installed on the second HDD?
If its already installed, just verify that the second HDD is connected, and powered on, and then run EasyBCD, go to Add/Remove Entries | Mac tab, select the EFI option, select the hard drive Snow Leopard is on, and simply create the entry. Then your mulitboot should work. Post back if you encounter any difficulties.
Looks like you have the wrong HDD first in the boot sequence in your BIOS. Reboot, enter your BIOS by pressing the key it tells you to press on the first splash screen you come to, move the Win 7 HD up to 1st place in the boot sequence, save the changes, and exit the BIOS. You should now see the menu with options Win 7, OS X, and Snow Leopard. Select Snow Leopard, and see if it boots.
The win7 drive is definitely first as it boots to the win7 bcd after the bios post.
The above screens are what occurs after i select snow leopard other two boot fine
I have changed the hdd order before to make sure that snow leopard boots up on it's own (and it does via the chameleon 2 rc3 bootloader installed on that drive.
I think i might open the pc up and swap over the connections of each drive so that win7 is on drive 0 and snow leo is on drive 1 and see if that alleviates the problem (no reason why it should but you never know)