I would suggest following this guide until you get a grip on Openstack as a whole. Do not set this up with xenserver first. Do a standard KVM install until it is work, _then_ begin setting up everything to work with xenserver. You will have a much better grasp of how to handle it as you will be going off book on a lot of things.

To actually answer your question, you would install the "controller" node inside of one or more DomU's on one node. Then you install nova-compute in a DomU on the other node. nova-compute talks back to xen to configure and set everything up. This is confusing at first, but once you get your head around Openstack it starts to make more sense.

But again, my advice is to follow this guide and stick with KVM unless you really, really have a reason to move to xenserver. I have setup both, and the KVM setup is much, much simplier.