This is currently an issue with editions of Windows that have Service Pack 2 installed (including XP Home, professional, and Professional x64). According to MOTU, SP2 technically does not allow for multiple hardware devices to be daisy-chained together via firewire. However, there are a few workarounds:
1. Uninstall Service Pack 2, or reinstall Windows with Service Pack 1.
2. Go to a non-64 bit edition of XP.
3. Use the optical ports (if available) to send ADAT out of one interface into another. This will allow for a full track count while only having 1 unit in the Firewire bus.
4. Check with the manufacturer. Some units have specially written drivers to handle this type of setup.
NOTE: This technicality does not apply to chaining separate devices together, such as a hard drive to an interface. It only applies to multiple interfaces on the same Firewire line.