Last night I recorded audio track 1 with a mic plugged into channel one of the Saffire, and the
track input was assigned to Stereo IP 1, and the signal was recorded on both the left and
right sides. It was perfect.
 
Oddly enough, when I moved on to record audio track 2, I set up the track just like track
1 but the recorded signal was only appearing on the left side.
 
I checked the assignment for the input signal, and it was assigned to Stereo IP 1, and I checked
the Saffire control panel and nothing had been changed.
 
Am I missing a simple principle here? Why would track 2 record only on the left side, when
I assigned it to Stereo IP 1, not Left IP 1 or right IP 1?
 
 

Check the track interleave in the Track Inspector.
 
You should of course be recording a mono signal into a mono track - set the interleave & input assignments accordingly and you'll be golden.

With a single mic you should only be recording mono.
 
If you are recording on input one you should use the left mono source.
If you are recording on input two you should use the right mono source.
If you are recording stereo (input one and two, e.g. two overhead mics) you should use the stereo source.
 
If you need to stereo reverb on top of this set interleave to stereo. So you are inputting in mono and outputting in stereo. Often it's best to send to a stereo bus though.
 
Another tip... Make sure you are running mix control 3.6 from Focusrite's site. If you are upgrading it is best to uninstall mix control first and reboot.

Your doing it in an unconventional way and it's easy to get confused at first because for some dumb reason stereo is the default and then add to that Mike 1 left is input 1 but Mike 1 right is input 2. 
If your mike is always pluged into input 1 then always select the very top choice, Mike 1 left mono. 
If you use input 2 then- Mike 1 Right mono. 
If you use input 3 then- Mike 3 left mono
If you use input 4 then -Mike 3 Right mono- and so on. 
 
Rule #1- Always record a mono signal to mono. 
Very few things get recorded to stereo. 
The ASIO input list will not read as I have stated. It might use "Instrument ASIO focusrite left mono"  etc. 

Thank you, to all of you! I really appreciate it. I was assuming way too much, and not paying
attention to how Sonar handles mono and Stereo.
 
I think I understand it better now, so I'll try it again and pay attention to mono-to-mono inputs!
 
Again, thanks SO mucH!