How to Connect two Domains on VPN

Hello All Dear

i have a domain A in head office and domain B at Branch Office. Having Tow Real IP and VPN connectivity already, now i want to connect both domains, i mean both domains share resources and user of Domain A can authenticate in in domain B and vice versa. both domain users must authenticate by their local domain controllers.
actauly we have a software that required head office domain controller authentication.

both office have different domain names and domain controllers
thankx in advance

zafar