Just to be clear, I think the steps would go something like this...
1 - Resize your current Lion partition say 500 GB down to 250 GB
2 - Create a new 250 GB partition in the space you freed up above and make it HFS+ Journaled and name it SL HD
3 - Boot from SL DVD and install to SL HD
This should work but you are messing with your internal bootable disk and if you make a mistake, it sounds like you didn't create the SL recovery USB when you had the chance before installing SL so you might be in a bit of hurt. For that reason, there's always...
1 - Attach a new USB drive
2 - In Disk Utility, change it from fat 32 or whatever to GUID, 1 Partition, HFS+ Journaled and name it SL HD
3 - Boot from SL DVD and install to SL HD
Whenever you boot, hold option and pick Macintosh HD or SL HD as you see fit. No need to touch the Lion recovery partition.
As for can't downgrade? I find it hard to believe a person couldn't find a way to wipe all the partitions from a Lion install, including the recovery partition and go back to SL if they really wanted to but it might require something like drive genius to pull it off.