If you really really want to run both you could. you could make a script using Fanoush's kernel and initfs flasher. You would then in 2008 run a script to write out 2007 kernel/initfs and shutdown. Then when booting boot to 2007OS. When you want to boot back to 2008OS, In 2007OS you could then run a script to write out 2008 kernel/initfs and shutdown. Then select the 2008OS on boot.

This is not that hard to do. (Thanks to Fanoush) I just see no real need to do it?