Hi, Scott
From the command prompt, concatenating two or more gpx files into one seems to work ( copy a.gpx + b.gpx ab.gpx ), although there's probably a more elegant approach. You then end up with one file with multiple tracks that are not necessarily connected or even nearby but display all at once when enabled.
It would be slick to, say, press the Shift or Ctrl key, then left click highlight the desired tracks, right click, click "Combine" or "Box" or whatever, name a destination file and poof! one file, multiple tracks. Looks like it would be super quick to implement. Just a thought, but perhaps including the Merge and Network functions under the same select --> Right Click menu might be handy.
Thanks,
Bill