I have set up a percussion kit on a normal staff-line (5 lines). I have used two percussion instruments for one line.
How can I switch from one instrument to another? How does the playback know which instrument to play?
When you’re inputting the music, you will see that the name of the current instrument is shown above the staff. You can switch from one instrument to another simply by pressing the up or down arrow; if there are two or more instruments associated with the same staff position, you will cycle through those instruments before you move to the next staff position.
For more information on percussion input, see here.