On the Windows box, I can't get the FTDI driver to create two Virtual COM Ports. The first one appears when I plug in one Arduino, but when I plug in the second one, I don't get another COM port.
On the Mac, I haven't tried opening both serial ports without Max. I suppose I could use two terminal emulators and open one port with each.
I ended up using two Macs, and piping data between them using OSC/UDP... it's kind of stupid but it worked.
I'll experiment more and see what happens.