Can we use the multiple databases system concept in a single web application? Am building a simple web based expert application.I have created two databases in the back-end,One database holds user data and the other holds user activity data.Hence it is accessed by a single web app.[e.g Distributed and Parallel both use in single website for back-end]

My question: Can a single web app have multiple databases in the back-end?
I don't see why not.  You would just create two data sources and potentially two data access layers if you want to take it that far.  At least for .Net/C#/VB.Net this wouldn't be a problem.
I'm not sure I understand the benefit in this case but if you feel you have a good reason to separate the database, there's no reason it can't be done.