I'm using my focusrite 6i6 and would like to at times add a second one (a cheaper one for an extra couple inputs).
Is this possible and how would I do it?
Thanks, Paul

On a mac its easy, but not in windows.
 
ASIO only allows one device at a time.

I wonder, would it work to daisy-chain one interface into another? Run the outs from the second interface into a stereo input in the primary interface. Seems like that could work. I did this with an earlier incarnation of my DAW with a pair of mixers -- ran the outs from one mixer into a set of ins in the second (primary) one. That worked fine.

"Is this possible and how would I do it?"
 
Free Way: Possible good enough with this Tool from Vb-Audio : https://www.vb-audio.com/Voicemeeter/banana.htm
new included Network features.
 
another (no free but very good) way is DANTE ( Sound over the Network ) https://www.audinate.com/products/software/dante-virtual-soundcard
 
 

There are 2 problems: synchronization and drivers.
Even in case you convince Sonar to use all inputs simultaneously (f.e. by not using ASIO or with ASIO4ALL) you potentially get the whole range of troubles: latency, de-sync up to crashes.
 
If you still do not use SPDIF on 6i6 and can find a device with such output, you still can use the same setup as now, with the only problematic place is the stability of that other device clock and ability of 6i6 to use that clock. That can add  2 inputs only...
 
But with all respect, in your signature you mention quite powerful computer, why not get a new interface with the number of inputs you need?
I in fact has used several unrelated "interfaces" in parallel before, than I have chained 2 M-Audio interfaces. But... (a) they support that on the driver level (b) the result is far from 100% stable (c) after I took RME, at the same day I was convinced what I have tried to use before was a waste of time and nerves. Make yourself an xmas gift

Yes you can use 2 interfaces for live recording of a band. You use WDM or WASPAI mode. It's when your doing overdubs of pre recorded tracks that you must stay with ASIO mode to avoid timming offset issues. 
So you can use 2 interfaces just for that first session. After that then you need to stay with your main interface and ASIO. 
 
But how many more inputs are you looking for? If only 2, you could use the SPDIF inputs you already have. This will involve a stereo pre amp that sports SPDIF output. 
http://www.joemeek.com/twinq.html
https://www.gearslutz.com/board/low-end-theory/549457-2-ch-mic-preamp-w-spidif-out-list.html
 
 
But for the price of a pre amp you can buy a Tascam 16x08 or if you can find one used one of the older us1641, or us1800. I use my us1641 every day and there zero difference between it and my 6i6. RTL performance is the same etc. Tascam released new ASIO drivers a few years ago that made a big differance. The Tascam is your best bang for the buck input channel count wise. 
 
That gives you 16 channels and if you team it up with your 6i6 for 20 channels. That's a lot. 
 
 
https://www.sweetwater.com/store/detail/US16x08
 
If your not planning on using all the inputs at once then a small mixer is the way to go. That's how I have my set up. I leave everything pluged in and it all goes to the 6i6. But my mixer is digital so I am using the SPDIF. The Tascam would do the same thing it's just I have that set up in a different location where I'm not recording Audio, just editing and mastering. 
 
 

Last night I had the same question involving a Focusrite 18i8 and a 2i2. The 2i2 will be here next week and I'll experiment with it. I want the 2i2 because it's a 2nd generation and my 18i8 is a first generation which is causing some problems in my system. I've read that you can NOT use two identical units together as the driver can't distinguish one from the other. Two different models may be a different story I don't know. If all else fails though, I'll just use the 2i2 and sell the 18i8 cause I really don't need all those inputs.

This article is old but may help. Much of what it says has already been mentioned though.
 
https://www.soundonsound.com/techniques/using-multiple-audio-interfaces-together 

If you kludge together two audio interfaces... keep this in mind:
Unless the audio interfaces are BOTH running off the same clock, tracks recorded across the audio interfaces will drift apart over time (due to slight differences between the two clocks).
 
The only way to make this (effectively) work is if both audio interfaces have digital I/O.
You can setup one audio interface as the "Master" clock source... and have the second look to its digital input for clock. 

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