Combining two tracks from two GT31 units is as easy as concatenating them.... so says Manfred from www.gps-speed.com, and we believe him

Manfred says "just select the first SBN-file (File - Open - left click), then press Ctrl and select the second SBN-file. You will then be asked if the two files shall be combined (averaged) - answer yes and if both firmware versions match you will see the combined results (red circle) together with the speeds from unit1 (GPS1), unit2 (GPS2), and the difference (green circle) and the errors (black circle - only if you used SDOP firmware)"

Easy as that... Get the latest version of GPS-Results