Plug each one into a different USB port on the mac, then go to Applications -> Utilities -> Audio MIDI Setup and... um... it looks different now that I'm using Snow Leopard - anyway, you create a new "composite device" and you should be able to add both the actual devices to the profile. Then, in your audio software, you select the composite device as the one you're using, and the inputs and outputs line up sequentially, depending on which of the two was the first one you selected. One of the few boo-yahs that macs actually have over windows in this area, imo.
It pretty much works for me with two different firewire devices (MOTU Traveler + Echo Audiofire 12), but I haven't done it for a while. I can't remember if the devices clocked together through firewire (doesn't seem likely) or whether I had to use an external word clock and slave the devices to it (seems more likely) - the clocking thing may be less of a problem if the devices are the same - but who knows. If the clocks aren't synched, everything works, but you get static/clicks/pops.
