Multiple clients on a single Xvfb server/process?
Yes, you can have multiple clients per Xfvb screen, much as your X display can have multiple clients.
Remember that Xvfb is essentially an XServer without a screen, just peeling off the actual video card writing and manipulation.
I'm curious as to why you need this? We needed to run this for Java code that couldn't run headless. It needed a frame buffer to write to even though it was dumping to a file only.