DNS between to VPN networks

I have 2 Forests that are VPNed together and I can ping each others IP address. So we can get to each others resources, but we can not use host names like “STIMPY” but I can ping it’s IP.

I have added forwards on both networks DNS server. I have tried added a secondary zone in each DNS to the other one, but it doesn’t work. Says it can’t connect. But again, I can ping everything.

I need to make a trust between the two, but must have DNS setup first. What should I look at or try?

ps. The branch office DNS is on the VPN, D.C. and Firewall server and the main office DNS is on a D.C. hope that helps.