Sharing multilink modems in W2K

Office1 has 15 people, a W2K server and DSL. Office2 has only 4 people, want to share 2 modem lines connected directly to a W2K server in Office2. These 2 lines will call to the Office1 to get email and internet access. I was able to make this with only 1 line. How can I activate the multilink with 2 modem lines?