What you normally do to open your gmail account.
go to http://www.gmail.com ,enter username and password and you get into your gmail account.
what if you want to check your other gmail account.you will logout from first account and login to second account.
Friends you can open two gmail accounts simultaneously.
when you click on your image on the top right corner you will get an option to add account.
just click on that button it will give you the option to add a gmail account.Enter your email id and password and you are done.Now you can switch between your two accounts.
The first signed in account will be your default account and it will appear on the top of the list.When ever you will open gmail in your browser you will land in the default account.
To make other account default you have to sign out from all account and sign in with that account.When you click on Sign out button.You will be Signed out from both the accounts.
If you do not want to sign out and a sign in again with different email id just to make it default , you can create bookmark and switch easily between accounts.
Hope this is useful for you.