Each RT has to be on a separate part - i.e. piano, strings, soloist, etc. You then use Bar Settings (F5) to mute/unmute the parts where desired.

In you example, assign the harmonica to the Strings part and the sax to the Soloist part. Then use Bar Settings as described above. The Strings and Soloists I used as examples. You can use any of the parts for the task.

If you don't have enough free parts, there are ways around it. Let us know if you need this more complex solution.

R