For Android Users:  

If you are using an Android phone, click the settings button (gear icon) in the upper right corner of the screen, then go to the Community page to link your Google account. By logging into the same Google Play account on other Android devices, you can play the game on multiple devices.

For iOS Users:

If you're using an iOS device, tap the settings button (gear icon) at the top right corner of the screen, then go to the "Community" page to link your Game Center account. Log in to the same Game Center account on another iOS device, and you can continue playing the game on multiple devices simultaneously.