just we have small problem with DNN portals. we have two DNN websites one DNN 5 and 7.5 we need to make users of DNN5 able to login to DNN7 with same username and password.can we do this scenario in DNN?
This isn't something that is easy to do with DNN, you'll need to setup a database task/service that keeps the users, user portals, roles, and ASPNET_ tables in sync across the two installs.
That, or you will need to write a custom authentication provider for both DNN 5 and DNN 7 that use an different database to store user information.