How to combine files in Realspeed:

1. Open the first file as normal.

2. Instead of "opening" the 2nd file, import it instead, (go to "file" then "import")

3. go to "View" select "Divisions"

4. right click over "Track Files" and select "Create Combined File"

5. expand "trackfiles" (click the "+" sign), then select the combined file.

(Many thanks to Decrepit for posting this information)

How to combine files in GPSResults

select 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.

Select to "average results from two gps devices",
and select to "concatenate files"

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)