I've only just spotted this!
Given that your two PCs have static IP addresses and are peer-to-peer networked (a Cat 5e crossover cable is all you need between two Network Interface Cards), the Host workstation connects to the ISP in the normal way and receives its dynamic IP address from the ISP server.
Your Host Workstation acts as Server to its mate which is a Client.
I use Deerfield's Wingate Internet Sharing Software found here:
http://wingate.deerfield.com/
Version 4.5.1 is their latest and it's cranky on Win2K, but version 4.4 is solid on Windows 98. Costs a little using a Credit Card on the Web. (£20-£30?)
So, it's one Modem (or ISDN Adapter), two NICs, one X-over cable and the software. The ISP only sees one IP address so doesn't know you're sharing the bandwidth.
I've had four PCs sharing an ISDN connection (through a proper LAN) and it was acceptable. But it's obviously better to reduce simultaneous access if you want a fast connection.
This set-up is more for the convenience than anything else.