NT-domain via ISDN-Router to a Novell 4

I administrate a NT-domain with 20 users. One of our customers wants us to be able to log on to their Novell-domain using an ISDN-connection, router to router. We both use TCP/IP and have the same IP-subnet. Neither wants to change IP-adresses. Whatshould I think of and what actions should I take to get this functional. To get the points I need some kind of walkthrough on this. Configuration of the routers can I handle myself as long as I know what to configure them with.

Jesper