using OSX server in conjunction with Windows server We have a mixed shop, about 60% windows and 40% mac.  The Mac users use the Windows server for sharing files with all users and for access to the internet.  I would like to have a mac server for running backups of the Macs and storing Mac software.  Is it possible to configure things so that the Macs are connected to both servers and using each one for the above-mentioned purposes, using the same cables?  Hang one server off the other, maybe?
Yes - servers work well together and you could configure things in just about any manner you wish.

As long as you can map out on paper how you want things to connect, you can configure the server and clients to connect to either or both servers at the same time. You'll have different shares on each server for many cases, but you could have one directory run for both servers so people's names and passwords were centrally managed and not different on each server.