I've also requested this in the past, and still hope to see it implemented one day. Also, just to expand on the request a bit. I would like any implementation to include forcing the first of any such linked tracks to be where playback starts. in other words, if the second of a 2 track 'set' gets selected by a smartlist, MC should automatically put the first track before the selected second track so they always play together.
It was suggested elsewhere that 'required previous' and 'required next' fields could work for this. This would also allow more than 2 tracks to be automatically linked in this way. (very useful for Pink Floyd)
I will also add that any system would need to properly handle the case where one has several version of an album in their library, so the linking would have to be very precise to not force all versions of a track to play, for example.
Hopefully someday