As an alternative to ROM slots, what you can do is,
1. Boot into 1st ROM, configure and download all the apps you want.
2. Take a Nandroid of; /System and /data and optionally; cache. But I don't use the latter.
3. Wipe Data, System, Cache, and dalvik.
4. Install 2nd ROM, boot, configure, install all the same apps (just for now)
5. Take Nandroid of /system only
To switch to ROM 1, restore the backup of its /system (should only take about 6-10 seconds as opposed to flashing data too.
To switch to ROM 2, restore the backup of its /system ""
The reason you MUST install all the SAME apps on the 2nd ROM is to rebuild the necessary data folders.
To install a new app, boot to rom 1 and install it, then switch to ROM 2 and wipe cache and boot or vice versa.