This similar to another question I asked here but phrased a bit more specifically.

We have 2 sites (A and B) and 2 domains (New.com and Old.com).
Site A: New.com
Site B: New.com + Old.com

For our transition from old.com to only new.com, we need users to be able to use DCs on either domain.

Currently, we have a stub zone setup for new.com on the old.com DC. This allows users using this DNS server to ping new.com DCs, however the computers think they are on Site A not site B. This causes problems with group policy.

So, stub zone is wrong.

How should I set it up?

Can I just set up a primary DNS server for new.com on the old.com DC then enable zone transfers?
Thanks!