ADS trust between two own domains

I would like to set a trust between two own domains.

Example: the headquarter of domain “abc.com” is in London. The headquarter of domain “xyz.com” is in Zurich. Both domains are
independent domains. However both companies would like to developa mutual position of trust, so that users of the company “A” can access on resources of the company “B”. How can I solve that? Does somebody know a link where this is well described?
I tried to set trusts between these two domains, but they didn’t find toghether.

Ccould somebody give me tips?

With regards
Ramon