Dual network connections on one computer

A Doctors office with six computers are now networked with a Linksys 10/100 hub. They also have an ISDN router into the hub and all of the computers connect to a remote NT4 Terminal Server through a VPN connection. They would like to bring in another internet service such as DSL to connect to the whole internet. I think this requires a complete 2nd network but am not even sure if that will work. Any help would be appreciated.