Storing all data in a single db versus multiple dbs in Mongodb
Well, 2.2 will add database level concurrency (this is available for testing in the 2.1 development branch as I write this answer), so if you are concerned about lock contention between the two, then separate databases would be a good idea.  

You don't really mention what your criteria are here to make the decision, so the new 2.2 feature is the only relevant one I can think of - feel free to comment or expand and I'd be happy to answer further.