Connect two offices with different domains over WAN

I have a client with two offices both running their own 2003 server R2, may be adding two offices in the near future. For now existing WAN is using 2003 to handle VPN and they experience long wait times for log on, up to one hour. I am considering the use of two sonicwall TZ 190’s on each side to create point-to-point. This is a two part question:

1. Will TZ 190’s make a good WAN connection?
2. How do I join two different domain controllers? I really want one login from either location to see same resources.

The two current offices have approximately 25 workstations. There are about 40 on the road laptops some use VPN to transmit data some use email to transmit data.