Unfortunately, I think I found the answer here > https://support.squarespace.com/hc/en-us/articles/206543977-Moving-collection-items-between-pages#:~:text=It's not possible to move content between Layout Pages.,collection items between Squarespace sites.
"It's not possible to move content between Layout Pages."
Hopefully, Squarespace can add this in time because it sure wastes a lot of time when you need to merge content!