Setting up a single DNS server on two separate locations

A huge corporation has 40,000 employees with their own related IP addresses and it works with around 400 servers that consists of SMTP, POP3, File Transfer Protocol, Web and Dynamic Host Configuration Protocol in two places that are 20 miles apart from each other. The Dual T1 lines is accumulating a bandwidth of 3,088 Mbps that connects the two sites.
Each site consists of 200 employees and servers. It works with various separated subnet in a Class B address of the company. Now, I want to know what if I setup a single DNS server on two places, can someone explain how the two sends support with each other?
And also, what are the advantages that this setup can give for the users of the corporation? And can some one also please explain the virtual speed of a 100BaseT Local Area Networks that are used in each places as compared to the WAN that connects the two sites.
