It cannot be done directly, but here's my method:
1. Drag the two mono clips into the blank area under the last track in the Track Inspector (scroll to get space if required)
- This is easier than using Import and always puts the clips at the beginning of the track.
2. Set the outputs to the hardware outputs of your interface
3. Select the two tracks (Track number must be blue, timeline must be highlighted over full clip lengths)
4. Ensure no other tracks are selected!
5. Pan the two tracks hard left and right
6. Go to the track dropdown menu, open the Tracks > Bounce to Track(s) dialog
7. Set:
Destination: New Track
Souce Category: Hardware Outputs
Channel Format: Stereo
Dithering: Triangular
Mix Enables: Enable Track Automation and Fast Bounce
8. Hit [ OK ]
9. Delete the two mono tracks if you don't need them