You can do a dual boot.
In Lion, open Applications->Utilities->Disk Utility.
Click on the actual hard drive (not the 'Macintosh HD' partition).
Click on the 'Partitions' tab; then the + a the bottom of the 'Partition Layout'. Resize your new partition if you want.
Make sure the format is 'Mac OS Extended (Journaled).
Click Options and make sure the partition map scheme is GUID partition table. (can't remember if this is necessary).
Insert your SL DVD and restart. During the restart press and hold 'C' so it boots from the DVD.
Choose your new partition to install SL.
One word of caution, you may lose your Lion Recovery HD partition. If so, you would need to reinstall Lion on top of itself to recreate it.
In terminal type... diskutil list and see if you see that partition after your SL install.
Get a second and third hard drive. Seriously, you will get dual boot system, and if you get a big enough hard drive for both the second and first hard drive, you'll be able to backup both. See my FAQ* on backing up:
No need to erase anything. No need to do any risky partitioning. No matter what people say, partitioning can lose data by accident. Not worth it unless your data is backed up at least twice in anycase.