To add on to Brian's reply...
I agree with him that it sounds like you're the beneficiary of the remainder of the previous owner's data subscription. You can check to see how much time is left on that subscription via Menu > About > My Account. It will show you which subscription you he purchased, and how long before it expires.
Check whether it just says "Base Subscription", or if it also says "Includes Multi-platform add-on". I think to have two iPads connected, you need the multi add on. You can decide if you want to continue using two iPads or not, but the add-on lets you have up to 5 devices attached to your account, in any combo of iOS, Android, Windows, or iFly Portable devices. I think the Base subscription lets you have one iOS, Android, OR Windows device, plus one iFly Portable device.
Those other devices in the list right now aren't hurting anything, and you can just ignore them until your current subscription runs out. Whenever you're ready to buy a new subscription, you can do that from the iFly EFB Store webpage. Here's a direct link to the subscription options:
https://iflygps.com/Store/subscriptions.
Note that the iFly EFB website is a separate site from this forum and requires its own account credentials. Those are the same credentials you'll enter into the iFly app after your subscription purchase to get it connected to that subscription. As Brian explained, once you're own your own subscription, you'll only see the devices you've connected to it, so the devices from the previous owner will no longer appear.
Whatever you do, DON'T DEREGISTER your current devices until you have a subscription of your own. Since you don't know the previous owner's credentials, you won't be able to re-attach to his subscription after you de-register, so you'll need to have your own subscription to attach to or else you won't be able to update data when it expires.