2x Xtion and a Windows notebook (a VM having XP on a MAC) Each camera standalone works fine with IPsoft. Both connected IPsoft works with one and to the other says :ipisoft failed to set usb interface

Whats the solution to this problem?

Most probably this is due to both sensors fall into the same USB controller.
Try different USB ports for the second sensor.

Do you actually use a virtual machine with Windows, or boot directly to the Windows via Bootcamp or similar solution?
Virtualization may give additional hardware problems, and we cannot guarantee the software will work as expected.
Bootcamp is a recommended way to run our software on Mac computers.