dahjah
I've been using two interfaces since I started with Sonar. I have the VS-700 and the Focusrite Liquid Saffire 56.
I have it set up with the VS700 as the master, the 56 goes into the roland via spdif. In Sonar you use the digital 1 & 2 on your input channels to record.
In preference input devices the focusrite comes up as VS700 DIGITAL 1 & VS700 DIGITAL 2 1-2
VS700 DIGITAL 2 3-4 and so on
 
I'm using Asio

FWIW, That's effectively using the Saffire as a "Pre-amp and A/D"
 
ASIO only allows a single audio interface.
Some companies have an ASIO driver that allows using multiple (of their) audio interfaces... but these show up as a single ASIO device (with more I/O).

With Focusrite is possible via Adat or S/PDIF

I asked their technical team if i could link my LS56 and 18i20 up and they replied yes its possible if you set one as the master clock and daisy chain them via an optical/digital link.

Jerry

The 6i6 doesn't have ADAT so that wouldn't work. Interfaces with SPDIF usually only provide single stereo pairs in/out so that would only get you two extra mono channels in each direction.
 
I've used ASIO4ALL in the past to get this done in a pinch and it worked incredibly well but it ends up being a matter of crossing your fingers and hoping for the best given that you have so many parts that all have to do their jobs well for it to work. What ASIO4ALL does is utilize the devices' WDM drivers to provide a single ASIO virtual driver for the DAW to use. If your devices' WDM drivers are quality and you configure ASIO4ALL and the DAW correctly, it all can work great but that ends up being a big if. 
 
If you have the money to spend on an additional interface, the easiest and most reliable thing to do is just sell your 6i6 and upgrade to a single interface with more inputs/outputs. Keep in mind when trying to find the model with the appropriate number of analog inputs/outputs that Focusrite's model naming includes all digital and analog inputs/outputs so, for example, you only get 8 analog inputs and 10 analog outputs with the 18i20 (but it has ADAT so can be expanded in the future via something like the Focusrite OctoPre or Behringer's cheaper alternative).