i think it goes something like this:
the first part was to add each others dns entries as a secondary zone, right click forward lookup zone, new zone, standard secondary, fill in the other domains dns zone info (ie one of them in our case was ctk.local). do that for both domains.
then active directory domains and trusts, right click your domain, select trusts tab, add your trusted domains and domains that trust this domain.
then you create a group in each domain with the opposite domains user.
maybe test it first.
hope it helps.