On further reflection, this process seems to be working as intended. Though it takes a few steps, it's easy. Here's a two-channel MKV example...
1. Import the file to the bin but don't place it on the timeline
2. Right-click on it and COPY
3. Right-click an open area in the bin and PASTE (Clip)
4. Right-click the original file and SELECT AUDIO STREAM 1
5. Right-click the copied file and SELECT AUDIO STREAM 2
6. Place the original on the timeline
7. Overlay the copy on the timeline, just below the original
8. Either Hide the extra video track or right-click on the track, UNlink from audio and then delete the video.