USB HDDs are not a good idea. They are too slow for serious audio work. Is your expresscard firewire 800 or 400? If your card is FW400 and has two ports, you would be fine running two devices on the card. Technically they would be on the same bus (the expresscard bus), but the expresscard slot has enough bandwidth to support the devices.
The only problem I can think of is power requirements for running an interface or Liquid Mix on the expresscard. To my knowledge, the expresscard slot is not capable of providing power to firewire cards or the attached devices. However, if your devices do nor require bus power, then this point is irrelevant.
15" MBP 2Ghz Core 2 Duo 6GB RAM, 2 320GB LaCie Quadra Drives, MOTU 828mk3, Logic 9 on SL 10.6
"There is no such thing as an old song, just bad ones."