According to WinISD, 2 cubes sealed for a 12" XXX will yield a Qtc somewhere around .600, not a great idea imo. I ran one of my XXX 15's in 2 cubes sealed, per RE's recommendations, and I felt it wanted abit bigger box. It needed more power though.
Sealed should be fairly easy to decide what you want. Plug the t/s specs into a program and figure out what alignment you want (alot of info on sealed alignment properties, use the search).
Hope this helps.