Does Exporting Busses in LPX Work?

I can't successfully get stem-exporting to work in LPX and I'm wondering if there's something I'm doing wrong.

I followed the steps in http://support.apple.com/kb/HT3650.

I'm trying to export two stereo busses, each of which has two tracks. During playback, everything sounds fine, including when I solo each bus.

However, each bus exports with only one of its tracks included.

Any ideas would be appreciated.