I've not tried it with the N900, but it should be possible (I've done something like this with the N800).

Search for "boot from SD card" for the second installation.
You'll likely need U-Boot to be able to switch between the kernels.

A quick search resulted in this recent thread (which seems to be exactly what you are looking to do):
http://talk.maemo.org/showthread.php?t=83719