My parents shared an iPad for years, but just now, they bought another one. They both had notes on the first iPad and wanted to share the notes accross the two apple IDs. I shared about 19 notes from ipad#1 and tried connecting with ipad#2. The first note went ok. On the second note I got an error message stating “***Can't Open Note Sorry, this note can't be opened right now. Please try again later.”*** This made absolutely no sense to me. After an hour of testing this and that, googling for solutions - this is what worked for me.
# Add note#1.
# Go to iCloud settings and turn off Notes + Delete from My iPad - then enable Notes.
# Add note#2
# Go to iCloud settings and turn off Notes + Delete from My iPad - then enable Notes.
# Add note#3
# (repeat)
This absolutely meaningless solution seemed to be what needed to be done to get it working. Both iPads were on the latest iOS (15.2)