An easier way is to use the export or mail function to export the files as GPX. This produces files that can be edited in a text editor, e.g. notepad, so much easier.
Open the first file, and you'll notice the actual track data is between two 'trkseg' markers, e.g.
<?xml version="1.0" encoding="utf-8"?>
<gpx xmlns:xsi="
http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="
http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd" version="1.1" creator="SkyDemon Plan v2.6.3.0" xmlns="
http://www.topografix.com/GPX/1/1"> <trk>
<name>Northampton Sywell - Northampton Sywell</name>
<trkseg> <<<<<< =================
<trkpt lat="52.301990" lon="-0.795403">
<ele>121.4933</ele>
<speed>15.844889</speed>
<time>2010-10-16T09:56:00Z</time>
</trkpt>
.....
.....
</trkseg> <<<<<< =================
******* INSERT HERE *********
</trk>
</gpx>
Take the next file, and copy the data between the <trkseg> markers. Paste it where shown. You can do this as often as you like, but make sue you keep the data in the right time order:
<trkseg>
.....
.....
</trkseg>
<trkseg>
.....
.....
</trkseg>
<trkseg>
.....
.....
</trkseg>
<trkseg>
.....
.....
</trkseg>
.gpx format files can then be viewed in programs such as google earth.