So to finish Jack's comment, don't copy data to the second table. Build a field in the second table that you can use to reference the related record in the first table. Then put the first table's unique record number in that second table's linking field where appropriate.
This does not work correctly, however, if the relationship is casual rather than formal and enforceable. So you need to specify whether there can ever be cases in the second table where there is no corresponding information in the first table to be linked.
I.e. Jack's suggestion and my amplification thereof only apply if the second table is a formal subset of the first one. If this is not the case, you need to let us know that in your problem description.
I'll answer ahead of time... if the two tables do not have a subset & superset relationship to each other, the answer is... you really can't do that in Access without a LOT of work restricting what your users can see.