You may have several email IDs and you may have 2 or 3 Gmail IDs. But you may not be able to login to gmail with 2 addresses simultaneously. This is because Gmail program or any other online application with login module does not allow 2 user login at the same time.
I have 5 Yahoo IDs, 2 Hotmail, and several Gmail IDs.
I found a way to login at least 2 Gmail IDs at the same time by using 2 browsers. Use IE if you have and login with 1 Gmail ID. Use firefox and Login with another ID. You can also use Chrome and login with another ID.
So this is the only way that you can use 2 or more Gmail ids at once.
If you have any questions or suggestions, Please post a comment.