How do I log in on an other device in SimCity Buildit

Can't speak for IOS but with Android just download and install with the same owner ID. Make sure you've got the original game backed up with Google Play or Facebook in the original device. DON'T try to play the same city at the same time on 2 x devices.

After you downloaded the game, you have to go through the tutorial again, and at the end of that it will ask your to name your City, and if you have used the same credentials it should sense it and give  you the option to download saved game from the Cloud. Hope this helps’