What the hell happened? I had two mics positioned above my drums and set them as a stereo pair in my Roland Studio Capture. I assigned them to a stereo track in Sonar X3. I tapped on the mics and could see (and hear) the level on one or the other side, as expected. While recording, it sounded fine.
 
Now I'm home, listening back, and the overheads are playing back mono, as if both sides were panned in the center.
 
I must be doing something wrong. What could it be?

Hi. I do not have a Studio Capture, but can you separate the ins (in your Studio Capture) and choose mono 1&2 if that is the inputs you wish to record to mono? The bleed between the mic's (in my humble / shallow mind) could cause you to phase out the stereo of your recording, when using one stereo track. Others may probably correct me 
I hope it helps on some level
All the best.

Yes, I could have just recorded each mic to its own mono track and then pan them left and right, but I didn't.  And I'm sure it was recording fine, with a nice stereo image. Something is happening in Sonar (I think) that is causing the stereo track to lose its stereo image.
 
The mics were positioned well (XY), so there wouldn't be any phase issues between them.

Maybe try to bounce the stereo track to dual mono? It might help.
 

Stereo interleave button accidentally set to mono?

No that was the first thing I checked.

Did the track you recorded have left and right channels, or was it a mono file? I am confused on this. It seems you have a stereo file that "sounds mono"?
 
*If* both channels exist and there was not a great variation between the two, it may sound mono (just louder). Ken's suggestion of bouncing the track and splitting into mono tracks would be a simple check... after doing that, you could phase reverse one mono track and see if they null each other out. If they do not, you have both overheads captured; and it is easier to pan them as separate mono tracks.

How do I split it into two mono tracks?

1. Make sure the audio track is selected (if only one wave file, can just click on the wave form). The track number highlights blue if selected.
2. Tracks menu (at the top of the Track View) - > Bounce to Track(s)...
3. In the pop up that comes up, it should default to "Destination" of New Track at the next available number. And in the right side of that window choose "Split Mono" under the "Channel Format" option.
4. Select "Okay" and it will bounce the original stereo track to two mono tracks.

Quick Edit - You may want to disable most of the "Mix Enables" at the bottom of that pop up, as it will bake in mutes, FX, etc., which you would not want. I would only leave "Fast Bounce" checked.

Didn't make any difference. It's as if both mics were assigned to both sides.