Moved to root forum for better visibility.
What you do in one ROM slot is totally separate from the other slot(s). So yes you can install different ROMs, or the same ROM over again and try different mods in it.
For instance, you have your favorite, well running ROM in slot 1.
You install something new in slot 2 and mess around with it for a while (hours, days, weeks, it doesn't matter). If you don't like the results or it doesn't work correctly you just go back to slot 1 which set up exactly like you left it.
Note that the ROM in any slot not being actively used is dormant. Any messages, for instance, that you sent while in a different ROM will not be in the other ROM(s).
I believe there are some limitations about which ROMs you can mix, you'd want to read the Safestrap thread to make sure. For instance it seems to me that the ROM slots share a kernel (though I could be wrong about that), if so then you'd have to make sure that the ROMs are both compatible with the kernel you're using.
Others who actually use SS will likely chime in to help you out.