Apple TV purchased Apps on second Apple TV Unit

so it is tied to the unit

No, subscriptions are always tied to the account that was used to start the subscription, not tied to any unit.

Note that Paramount+ has several ways to subscribe to their service. Which type did your s.o. select?

• Users can start their Paramount+ subscription in the Apple TV app. Then also watch this service in the Apple TV app as in-app Channel, when signed in with the same Apple ID account for the device or app.
• Users can start their Paramount+ subscription in the Paramount+ app on an Apple device. Then also watch this service in the content provider app on any of your Apple devices when signed in with the same Apple ID account on that Apple device.
• Users can start their Paramount+ subscription directly with Paramount+. Then watch this service in the content provider app and sign in with the Paramount+ account as used for this subscription.

These types of subscriptions have different terms and conditions, different perks and limitations, and are (mostly*) not exchangeable as such.

Subscribe to Apple TV channels in the Apple TV app - Apple Support

Apple TV Channels FAQ: Services, pricing, availability, and everything else you need to know - Macworld (US)

Paramount+ app - App Store preview - Apple

I know this is an issue because of trying to use my ipad to cast to my older Apple TV that does not support the apps i need.

Some content providers do not support AirPlay, or even block AirPlay, or have minimum system requirements for the receiving device. That is their prerogative. See their respective websites for details on AirPlay support. A current model Apple TV wth all the apps natively supported would solve just about all of that.