Hey there Phanomis,
Welcome to the Chromecast Community! Currently, the way to have the Chromecast with Google TV choose the account to use each time is to make sure that before you shut out of the Chromecast, that you hold the Home button, select Switch Accounts, and then turn off the device when you are on the screen where you choose an account. Then the next time you turn on the Chromecast, you'll be met with that screen.
I hope that helps you out, and if you had any other questions or concerns, please let me know.
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Thanks for the help, Jeran.