You may partition your HD and install 10.6.8. on the second one. So you'll keep your exisiting Lion alive and have the choice.
The Disk Utility will not erase your Lion partition when adding a second partition for SL.
Make sure you have a 10.6.8 to start your iMac 2011 with. Earlier versions might not work => kernel panic.
The discs from your other Mac will not do the job; they're machine-specific. Nor is the retail SL disc likely to work as the original OS would have been later than 10.6.3 (hence alquimista's notes).
You may be able to get the original SL install discs that would have shipped with your new Mac had it not had Lion installed by calling AppleCare/Customer Services with the serial no. of the Mac.
Others appear to have done this satisfactorily.