Merging 2 AD domains into a single AD is actually a little more complicated than changing their layer 3 addressing schema. I assume this is something you've had experience with before? really, you can screw up all kinds of things doing something like this if you're not careful. I would probably just make a trust relationship between the two domains and leave it at that.
but from a networking standpoint, you want a tunnel. In terms of business class, you want something with an SLA like a frame relay/ATM or PTP Ethernet. If you want to just bank on the internet connection you have, a better solution is to have seperate boxes create a point to point tunnel between the two networks and create static routes between the two disparate subnets. You could even do a router-to-router tunnel to cut down on the costs. OpenBSD has the software to do this out of the box. If you google "openbsd isakmpd tunnel" that should point you in the right direction. essentially, all packets destined for the remote network traverse the packet and higher level protocols/apps will see it as a locally attached network segment.
