If anyone could help, I'd sure appreciate it. When I go into Preferences | Audio | Devices, Focusrite devices show up, but they are grayed out / disabled, and I cannot select them. I tried unselecting the E-MU ASIO drivers, thinking that maybe there was a limit to how many could be selected, but it made no difference. I've tried it both with Scarlett Mix Control running and without.
 
Any thoughts?

There is indeed a one device limit for ASIO. Make sure Sonar is in ASIO mode (not WDM or MME or whatever else is available) and make absolutely sure there are no other interfaces selected in your audio Preferences.
 
You may want to try closing, waiting a minute, then reopening Sonar after disabling the Emu (and all other devices) in Preferences or maybe even rebooting. Then open Sonar and see if the 18i20 becomes available/not greyed out.
 
Sometimes Sonar hangs on to stuff/won't recognize stuff until you close it. I get this with MIDI gear all the time.

And if all else fails you could try uninstalling and reinstalling the drivers. That's a last resort though.
 
And just to be sure... you downloaded the latest drivers from the Focusrite site, right? Don't use the disc. Those are usually outdated.
 
Good luck.
 

I am not sure if this has changed, but shifting devices I will first shift SONAR to WDM mode (so everything Windows "sees" is available), then select the desire device and shift back to ASIO mode.
 
Another side point is to check Windows sound card settings, select the Scarlett, then properties button. On the Advanced tab, set the default format to what you normally record/work in, and uncheck both "exclusive mode" settings. Do this also for the Recording tab. This will prevent Windows from interfering with SONAR when trying to adjust things within SONAR itself.

Thanks to all of you, I have it working now. I DID shift to WDM mode and clicked apply, but I'm not sure it was what allowed me to select the Focusrite. What I THINK did it was unselecting all the EMU devices (both input and output) first and clicking apply (I had only tried inputs the first few times). It then let me select either EMU or Focusrite ins and outs.
 
Thanks again for your help!

Yes, deselecting both ASIO input and output device settings is necessary. The ASIO spec restricts the host to one I/O driver at a time.