When you want to design a customised outing and look for tracks in the area, it’s difficult for these to be perfectly adapted to the distances that you want to cover, the points that you want to leave from or arrive at, etc. In this case, the best idea is to join multiple tracks and/or cut them at the point of interest to you.
Let's see the steps to follow:
1- Load the 2 tracks you want to mix.
2- Decide the portions of the tracks that you want to keep and those you don't.
3- Cut the tracks so as to separate the "unwanted" sections. To cut a track simply right-click on the cut point and select "Tools > Cut track here". So, you'll have to "Cut track here" at the beginning and end of the sections that you do not want to keep on each of the tracks.
Note: Each cut, technically, will generate a new track for you, so you will see new tracks on the data tree.
4- Close the sections of the "unwanted" track. It is best not to close them on the map: i.e. right-click on the stretch of the track on the map and select "File > Close track." This will load only those track sections that you want to form part of your final track.
5- Go to the main menu and select "Edit > Unify tracks". You will be asked if you want to join them by proximity or time. In this case, the most advisable course is based on proximity. In this way you'll have your unified track all ready.
Note: You can also decide whether to keep the two parts of the track marked "Section" on the final track. For simplicity's sake, say no.
6- The unified track may have a starting point that you you do not want. Selecting a new starting point is very simple: just right click on the new starting point and select "Tools > Start point here".
7- Don't forget to save the final track!
Important: As always, before starting to edit files, you should make a backup, since it’s easy to make a mistake when modifying them.