Rails: mixing NOSQL & SQL Databases

I'm looking for the better way (aka architecture) to have different kind of DBs ( MySQL + MongoDB ) backending the same Rails app.

I was speculating on a main Rails 3.1 app, mounting Rails 3.1 engines linking each a different kind of DB ...

... or having a main Rails 3.0.x app routing a sinatra endpoint for each MySQL/MongoDB istance ...

Do you think it's possible ..., any idea or suggestions ?

I notice some other similar questions here, but I think that "mounting apps" is moving fast in Rails 3.1 / Rack / Sinatra and we all need to adjust our paradigms.

Thanks in advance Luca G. Soave