Is it possible to create a second fork from a repository using same user as the first fork? If it is possible what will happen to the first fork once the 2nd fork is created?
It's not possible to create two forks of the same repository in the same user account, no. Although, it's rarely necessary to do so since if you need to keep two different repository states around, you can always use git branches.
On the other hand, you can create a single fork and then create a non-forked copy of the repository. The copy won't have the easy support for pull requests though.
If you can give some more details as to what it is you're trying to accomplish, I can probably give some more advice.
Let us know!