There isn't a lot of documentation out there that covers deploying Team Foundation Server across two non-trusted domains.  This is not to say that TFS simply won't work this, but that there is simply not a lot of deployment documentation to support this scenario. There is an MSDN forum post (http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=177844&SiteID=1) that discusses it, but no formal guidance. 

The below is a diagram outlines a deployment configuration that should support this scenario.  Again, it's not formal documentation, but rather a visualization of a workable scenario. 

The full-size image is attached to this post
The full-size image is attached to this post.