You can playback more than one GPX file at a time:
start a gpx playback
manually move forward the progress bar
on the map display, select the upper right 'Menu' button
restart the same gpx playback
watching the progress bar, you can see it process two positions
at this point, even if you use the 'Stop' button on the right of the progress bar, the first gpx replay is still running and there is no way of stopping it.