Mapping: Static IP Needed for Second MU Site?

If I set up a second WPMU installation with mapping on the same VPS server that already has a a WPMU installation with it's own static IP and mapping , do I need a second static IP?

Background: I have a VPS server supported by a very good hosting company. I have a WPMU installation with a fixed IP and domain mapping enabled which I set up last November.
I've recently installed a new WPMU installation on the same VPS which has its own static IP assigned.

When I go to activate the domain mapping plugin (v4) on the new WPMU installation, the setup detects the old static IP and instructs me to use it rather than the new IP which was been assigned to my new WPMU.

Which one should I use? Do I even need to static IP's?

I'll attach a screenshot.