If you link to your existing Jeppesen account to your ForeFlight account on the More > Jeppesen page of the app or on the Jeppesen page of your account on ForeFlight on the web, each iOS device on which you want to active the Jeppesen coverage in ForeFlight needs an available Jeppesen license (seat). Activate that coverage on the More > Jeppesen page of the app by tapping "Install Coverage."
Jeppesen App coverage purchased through ForeFlight is automatically made available in ForeFlight on all of your signed-in iOS devices.
If you have both a linked Jeppesen account and Jeppesen App coverage purchased through ForeFlight, a device on which you have installed the coverage from the linked account will have the combination of the two Jeppesen coverages.
You can also now purchase Jeppesen coverage for use on ForeFlight on the web, or for both in the App and on ForeFlight the web. NOTE: Jeppesen coverage from a linked account is not able to be viewed on ForeFlight on the web at this time.