What do I do if I have a single pilot subscription with Jeppesen but I don't have a ForeFlight subscription?

You will need to contact Jeppesen to cancel your current subscription. You will be able to purchase ForeFlight and your selected Jeppesen chart coverages in one transaction via the ForeFlight website. With this option, you will have the ForeFlight individual license model that will allow you to use Jeppesen charts on all of your allotted devices (one iPad, plus one backup iPad, and one iPhone).