KawLiga, it could be very messy but you could do it. What you would need to do is get a software that allows you to create a "Network Profile", this way you have to coordinate with the user of the other machine and decide who is going to be the server this time around. Once having decided, you use the "server profile" and then login, while on the 2nd machine you use the "client profile" which and switch the next time..
This can be rather a lot to do each time you wanna get onto the 'Net. The easier bet would definetly be to just go with a single server and a single client scenario so that you aren't switching profiles. The newer PROXY/connection sharing software are smart enough to realize that when you are using a dialup connection, if the connection has not been made and a client attempts to access the Internet, it will automatically dial to your ISP and make the connection..
This is a better path for sharing a single 'Net connection, regardless the type, IMHO..
Regards