[Zope] multiple zope instances on one ip address

I am trying to set up more than one zope instance and failing... I have configured two seperate installations (different versions, in fact) of zope, and lets say I try to run one on 192.168.0.1:2000 and the other on 192.168.0.1:8000. If the port 2000 one starts first, it will also answer requests for port 8000. If I then kill the port 2000 one, the port 8000 one will then be able to answer requests. I want both to run without interfering with each other. BTW, I start them with: firstinstance/z2.py -X -w192.168.0.1:2000 -u nobody secondinstance/z2.py -X -w192.168.0.1:8000 -u nobody If this just the way things are, or am I missing something? thanks!