Not necessarily. If you are careful, you should have no problems with 2 people playing on the same device with different King or Facebook accounts.
To do this, keep these steps in mind when switching between players:
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Make sure that the last person has signed out of their King or Facebook account in the game.
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Force close the game.
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Wait a few seconds, and then reopen the game.
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Then, sign into your own King or Facebook account and let the fun begin!
If you have the Facebook app installed on the device, be sure to use the 'Login with Phone number or Email Address' option when signing into the game to avoid the game defaulting to the account linked to the Facebook app.
As long as each player logs out after playing and the next player logs in with their credentials, each player will have their progress saved and can continue playing on the same device.