Assuming the modems are properly configured for leased line usage and that RAS is already installed:
Control Panel / Modems / Add
Check Don't detect my modem, I will select it from a list and click Next.
In the Manufacturers box select Standard Modem Types and select Dial-Up Networking Serial Cable between 2 PCs in the Models list , click Next.
Select the port and click Next and finish responding to the dialogue.
Now that the modem is setup, configure the RAS connection using Control Panel / Network / Services tab and select Remote Access Service, click Properties and configure RAS normally for this COM port.
When configuring a phonebook entry for outgoing use, check the Persistent connection box in the Dialing section.