For technical reasons, after the latest update, all farms running on iOS4.x, iOS5.x or iOS6.x are not able to be transferred to another device, unless the other device is running on iOS7 or later. It is especially important to install these updates if you want to use the same farm on more than one device. So please, update now if your device supports it.
Here’s how: http://support.apple.com/kb/HT4623
If your iOS device doesn’t support iOS7 or later (or you don’t want to update), you can still connect your farm to Facebook in order to play on more than one device.