How to set old iPad to factory default without losing info on my new ipad
Setting old iPad to factory default with losing information on another paired ipad
iPad Pro, iPadOS 13
Erasing one iPad should have no bearing on the content on the other iPad. If you have set up the new iPad you can safely erase the old one. Take a look at this.
What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support