Grade 9 Students,
To assist you with your mashups, you can use this online metronome tool to tap the tempo of the song you hear. This will give you the beats per minute (BPM) of the song.
When you do it for both tracks and they are the same tempo (for example, 113 BPM), then you can combine them in GarageBand.
MAKE SURE the beats line up. Even though the two songs might be the same tempo, they will be out of sync and the song's rhythm will be bizarre.
It take patience to do this - trial and error.