How to combine files in Realspeed:
1. Open the first file as normal.
How to combine files in GPSResultsselect file > open, select the files that you wish to combine (concatenate) with the <ctrl> key pressed and with the left mouse button click each file to select,
now select open.
you can save the combined file to sbp format via the file save trackpoints menu, selecting navi files sbp format.
(Many thanks to OPH for posting this information)