Usually, it should be possible to use BodyFast on two devices at the same time (for example iPhone and iPad).

If you have problems with the synchronisation, please test whether you are logged in on both devices with the same e-mail address (in the app as well as in your iCloud account). 
If not, log out on the device in question and press "I already have an account" on the start screen.

In case your data did not sync although you are using the same account on both devices, please log out on both devices from the app, restart the devices and log in again on both devices.

In case your Coach subscription is only active on one device, please make sure you use the same App Store or Google Play account on both devices.