One application that integrates with Visual Studio and allows collaborative code editing is Source Meet
Source Meet supports the collaboration of software developers in
project teams using Source Meeting. The basis of the collaboration is
provided by the familiar Visual Studio™ Editor, by means of which
spatially separated documents may be viewed and worked on together. As
not just the document contents but also the Visual Studio™ Editor
properties such as highlighting or positioning are synchronized in
real time, you get the feeling of sitting together in front of one
monitor.
