You can have the eKey app installed on multiple devices, but you will only be able to use one of them at a time. For instance, if you were using it on your iPhone and then wanted to switch to your iPad, you would have to get an authorization code for the iPad, at which point the iPhone would not work as your key until you authorized it again (then the iPad will not work).
Both devices cannot be authorized simultaneously. This is done for security reasons.
Click here to see a list of compatible eKey devices (as of 2/26/19).