I bought a used ipad (5th gen.) to dedicate running Pianoteq and leaving connected at midi piano location. You are suppose to be able to run the app from multiple ios devices as long as the Apple id is the same. First time attempting to login using second ipad I get error: "Authentication Failed". Why?

What version of iOS runs on it? 

This is worth a question to moddart support.  Include the version of pianoteq you are using, the  current iOS level (Settings -> General -> About)

I've generally gotten replies within 3 days.  Good crew there.

I bought a used ipad (5th gen.) to dedicate running Pianoteq and leaving connected at midi piano location. You are suppose to be able to run the app from multiple ios devices as long as the Apple id is the same. First time attempting to login using second ipad I get error: "Authentication Failed". Why?

I use Pianoteq on multiple iOS devices no problem.  What login screen are you talking about?  Are you sure your used machine is using the same AppleID?  Need more detailed info to diagnose your issue.  I assume you've downloaded Pianoteq and are able to run it in demo mode (i.e., with the 20 minute limitation) fine on the used iPad you got?