Aggregate Soundcard - Device not selectable

I have 2 external audio interfaces (Tascam US-1641 & Zoom R16) installed on my Mac (running 10.6.8 Snowleopard) both connected by USB.

They are both installed correctly and drivers are up to date.

I can see them in my audio preferences and each are selectable as my primary soundcard for input and output. They both work.

In my audio devices page in "Audio MIDI Setup", both devices are visible in the left column, however when I go to create an aggregate device, the US-1641 doesn't appear on the right as one of the selectable devices.

Does anybody know of any reason that a functioning audio interface would not be selectable for use in an aggregate device?