Running IIS and Apache on the same box creates a couple of challenges but the primary one is port conflict. When an http request comes in on port 80, which program handles the request?
Using your IIS for Outlook Web Access complicates the picture as does my experience that Apache on NT is not nearly as robust as Apache on Linux.
I think I would look seriously at seeing how hard it would be to port your Apache Web stuff to IIS and just put it as a virtual site on a single IIS box. As much as I love Apache, it doesn't do OWA and never will whereas most of what you can do on Apache, you can also do on IIS (although not as efficiently or elegantly).
paul