Two Usb towers

My current setup use's two IR towers, one serial, one Usb, to communicate to two
Rcx's. The usb setup is contained inside a cardboard box to prevent clashing of
the IR.

I'd like to swap the serial tower for a Usb version so I don't have to keep on
having to buy 9V batteries.

I have a Visual basic program that talks to the two towers, and it's really easy
to differentiate between them, but if I use two towers I'll need to address Usb1
and Usb2 via the Phantom.dll. Is this possible and if so which command(s) do I
use to differentiate between the two towers?

Thanks

Steve