Tracks are merged (mixed) just by being one above the other. So you could in fact retain two tracks and insert silences and repeats in each. Press F5 to time shift one track to start after the other, then F1 to go back to selection tool.
Does this Nyquist plugin help:
https://forum.audacityteam.org/t/pauken-add-silence-and-repeat-chunk/29659/8
Or without making repeats:
https://forum.audacityteam.org/t/inserting-pauses-to-aid-transcription/32077/1
Or without making repeats but inserting silences depending where the existing pauses are:
https://forum.audacityteam.org/t/extend-silences/34612/1
See How to install Nyquist plug-ins.
Gale