Hi
If you create a SEQUENCE of your clip(s) that has two Audio tracks, VP will combine both tracks into a single Audio Track. The sequence will now be treated as a single clip with one audio track. Loading the sequence back to the timeline will only show a blank Audio Track1 although when played the sound will be present.
The "Drag and drop your audio clips here to mix" Audio track 2 is no longer visible; or indeed seems to be available for dragging further audio clips onto although more audio tracks are actually present.
To proceed....
Drag and drop your new audio file to the apparently blank Audio Track 1. Here it will be visible as a waveform and mix with the unseen audio of the sequence.
Once you have done this the "Drag and drop your audio clips here to mix" prompt will re-appear below Audio track 1 and you can then simply drag your new sound down from Audio Track 1 to the now visible Audio Track 2.
Audio track 1 will remain apparently empty, but you can, if you wish still add audio clips to it instead of using Audio Track 3.
Nat