Firefox has a very cool property: you can run it in multiple instances at the same system using different user profiles. I've made a mod, which does that. Please, add it to watin core. The files changed are attached.
Here's an example, how to use it
Settings.AutoMoveMousePointerToTopLeft = false;
Settings.CloseExistingFireFoxInstances = false;
FireFox.PathToExe = @"E:\program files\Mozilla Firefox\firefox.exe";
FireFox.Port = int.Parse(args);
Browser b1 = new FireFox("", args);
this can be run in multiple instances using pre-created facebook profiles to launch independent firefox instances.
Let me know, If I can help you better integrate that into the main watin core.