Sorry if this has been asked before, but I’m a new ios and atv4 user as of today 
If I want to get Infuse Pro version on both my ipad and atv4 do I have to pay for it twice?
If you use the same iTunes account then you only need to purchase it once to use it on any iOS or tvOS device using that account. This is true for both the direct purchase of the Pro version in the App Store, or the free version with the in app Pro upgrade.
I would recommend that you purchase the version that is advertised as the Pro version in the App Store, rather the the free version with the in app Pro upgrade.
While both methods result in the app supporting the same features, and it is the same cost, due to Apple’s Terms and Conditions you cannot share in app purchases using the Family Sharing.
So if you buy the Pro version directly from the App Store you can share the Pro version using Family Sharing, but if you buy the Pro version by an in app purchase then you cannot share is using Family Sharing.
Thank you for the detailed answer!