how to have TWO unique fields on registration?

I have created a registration system where the user must enter email, password and gamertag.

The password and email will not be displayed to other site users. Only the gamertag. As such, the gamertag needs to be unique.

When a user signs up with an email that is already in the system it warns them - but, if they use a different email and a gamertag that is already in the system there is no warning. So, there can be multiple users in the system with the same gamertag.

I don't want to use the gamertag as the login (because they forget what they made up) but I need them to enter a UNIQUE gamertag when they register.