When you want to make a mashup, you need to add the tracks that you want to use to your Mix.
DJ.Studio will automatically put those tracks in your mix like a DJ mix.
The transition window is used to go from one track to another.
But if you want to make a mashup, you don't want to have the tracks behind each other, separate by those blue transition windows.
If you want to make a mashup of two tracks or make a redrum you follow these steps:
add the tracks you want to use to your Mix project
Put the playhead in the second track
Press Ctrl/Cmd+A to select the whole track,
Press Ctrl/Cmd+L to copy it to the sample lane and move the sample below the first track.
OR
Choose one of the options to copy one of the stems of the track.
Eventually, when you are ready, you can put the playhead in the second track and click Del to delete the track.
This video shows how that works: https://youtu.be/IYTXwoCNTAU