Seperate login but combined logout
It's one of the weird things about the skylogin feature.
Only thing I can suggest is keep a separate browser tab open and logged in for My Account and don't logout. In email just close the tab email is opened in, the tab that My Account is opened in should stay logged in.
Unfortunately, anytime you log out of anything that is connected to the MyAccount page, whether it's email or the Community or the MyAccount page itself, it logs you out of everything. The only thing I can suggest is like BirdDog mentioned, and that's just closing (X-ing out of) the email tab/window instead of actually logging out of it. It will still close your email, but it shouldn't log you out of anything else. But, the second you log out of your email it will log you out of everything else connected to that MyAccount page, including the page that includes your data usage, as it's all part of the skylogin.
