Windows
The FA-66 and FA-101 supported two standard Windows driver protocols: ASIO and WDM.
ASIO:
Due to the industry standard nature of ASIO drivers, the FA interface can't be used with another audio interface simultaneously on a Windows computer when selecting ASIO as the driver mode.
WDM:
The WDM driver protocol allows for multiple audio interfaces to be used simultaneously. Check the documentation of the audio software that you're using to determine if it's compatible with WDM driver mode and, if so, how to setup multiple audio interfaces using WDM drivers.
If the audio software is not compatible with WDM, or if you would prefer to use ASIO drivers, customers have successfully accomplished setting up multiple ASIO audio interfaces with a 3rd party driver software program called ASIO4ALL:
www.asio4all.com
NOTE: Roland is, in no way, affiliated with this software or it's designers. We just want to offer a possible solution. Please contact the software designers/developers of ASIO4ALL directly if you would like any information regarding support and operational questions with this or other non-Roland driver software.
Mac OS X
The FA interface can be used with another audio interface simultaneously on a Mac OS X computer by creating an aggregate device to use multiple audio interfaces simultaneously. The following article guides you through creating an aggregate device and then setting it up in audio software:
Creating an Aggregate Device in Mac OS X