offline Replication

We are planning to build some system where there will be two different instances of databases will be running in the system. Where one will acts as Master db and other acts as throwaway db. User will do data addition on throwaway db. On need basis we need to replicate data from throwaway db to Master and vice-versa.

Does mongo provide such replication possibility ?

Sounds like you are looking for something like master-master replication. Master-master replication is currently (2.6) not available, but there is SERVER-2956 requesting it. You can follow the discussion there and watch, vote, and comment on the issue. For the present, you'll need to work out the reconciliation of the two databases yourself.

Well, I'm not sure what "offline replication" is or what you mean by "on need basis" but I would disagree with Will - this does *not* sound like master-master replication.   It sounds like you want to do your own "on demand" data copy from one to the other and vice versa.  It sounds like two stand-alone systems (or separate replica sets) where you need an extra layer that fetches some data from one to the other (in either direction).

Maybe if you explain in more detail what the purpose of this system is we can figure out if there is already a simple way to do this yourself or if it requires some future feature.