Personally, I'd just use one or the other. The Fireface series seems to work OK with Pro Tools 9. I'm currently running a Fireface 800 with Pro Tools 9.0.1 and 10.6.7 and it works without a hitch. The only issue is that most third-party interfaces can only go down to 64 samples (latency settings) in Pro Tools and that monitoring through Pro Tools cannot be disabled, so using the near zero latency monitoring on the RME units is hampered a bit. I'm not sure how low the M-Box 2 Pro goes, latency wise. The M-Box 2 Pro's converters are often said to be audibly inferior to the Fireface units, but I've never used one, so use your own ears and decide if it's worth the effort.

You can technically have both units connected at the same time, yes, but I'm not sure why you'd want to.