Index cards generally in Word are going to be cells in a Table.
Reordering those is not going to be simple, especially when you are doing both sides.
Probably the best method would be to be doing the cards as a label mail merge and re-ordering things in your data source.
- https://www.addbalance.com/usersguide/tables.htm
- https://support.office.com/en-us/article/mail-merge-using-an-excel-spreadsheet-858c7d7f-5cc0-4ba1-9a7b-0a948fa3d7d3
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If you have each card as a single page, then you run into the problem that Word Doesn't Know What a Page Is! https://wordmvp.com/Mac/PagesInWord.html
See Moving/Reorganizing pages on my website: https://addbalance.com/word/MovePages.htm#PageStart