Asio4all (http://asio4all.com/) can recognise an unlimited amount of devices/audio interfaces - then i think JRiver should be able to use asio4all as output
Asio4all is a workaround for interfaces without Asio driver. The standard and better solution is Asio which has been developped for recording gear.
Focusrite comes with Asio and the opportunity to link several interfaces as daisy-chain (one single FireWire port) or parallel connnected (several FireWire ports).
Excerpt out of the Focusrite Saffire Dual Unit manual:
Computer with a single FireWire port:
Connect one Saffire to the FireWire port on the computer.
Connect the second Saffire to the spare FireWire port on the first Saffire.
Note that the Saffire PRO 24, Saffire PRO 24 DSP and Saffire PRO 14 have a single FireWire port, so will need to be connected to a Saffire PRO 40 or Liquid Saffire 56 and will require the external PSU when connected in this way.
Computer with multiple FireWire ports:
Connect both Saffires directly to computer.
Note that both FireWire ports on the computer MUST be on the same FireWire bus. i.e. the same PCI / PCIe FireWire card.
When dual Saffire devices are connected to one computer via FireWire, the two devices will automatically sync together with no need for any additional cable connections. However one device will be set as the Master Sync Device.
Saffire MixControl allows control of two units from the same interface (as GUI). The input and output streams of both units will be combined into a single list when viewed from the DAW, or as seen by the operating system audio preferences when outputting sound from iTunes/Windows Media Player. Each connected Saffire will have a (fixed) hardware ID. Inputs and outputs from the Saffire with the lowest hardware ID will always appear first in the list.
Audio can be routed from Saffire to Saffire using your DAWs own routing capabilities. This allows audio processing from the DAW, but will result in audio latency due to the DAWs audio buffer.
End of excerpt
http://d3se566zfvnmhf.cloudfront.net/sites/default/files/downloads/7241/saffiredualuniteng0.pdfThere's no way to use two audio interfaces in a single zone at one time, unless the driver for the device can present them as a single device.
I interpret the answer of Matt that a Dual Unit concept works there the two interfaces are seen as one unit. Perhaps there are users with JRiver experience of such Dual Unit solutions.
Ruediger