Just bought a new iPad and all the info is now shared with my old iPad. How do I restore my old iPad to factory settings without changing anything on the new iPad?

.Perhaps if the old iPad were removed from the iCloud, then restored to original iPadOS.

..that would allow no other content to be imported or sync'd to from the new iPad..

[Isolate older one through settings, if needed; so it cannot get any other content later.]

These may help:

• Sell or give away your iPad - Apple Support

• Erase iPad - Apple Support

• How to factory reset your iPhone, iPad, or iPod touch - Apple Support

• What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

(Or maybe needed: Article more inclined suggestions 'how to restore' iPad)