Cutting back and forth between two clips?

I am a lower-intermediate level user of Final Cut Pro, although it's been a long time since I've worked on a complex project and I can't figure out the best way to accomplish the following.I am piecing together an event involving about ten different people. Due to COVID-19 concerns, each person came into the venue and recorded their parts of the presentation separately (multiple parts per person). I have been editing the entire event together for live streaming this coming weekend.It's been going together smoothly, but now I am stuck.The presentation involves some music. Up until now, the singer/guitarist has been the on-screen subject while she is performing. This is easy to edit into the production.Now, I have a sequence of a couple of minutes when I need to cut back and forth between her singing and playing and another person (recorded on the same camera, but at a different time) doing something else. I need her singing and playing to be heard exclusively, and without interruption, even when the camera cuts away to the other person doing their thing. It needs to look as if the two of the were in the room together, doing different things simultaneously.I tried stacking a secondary clip of the singer above the main timeline clip of the other person going through their motions, which resulted in the singer and her soundtrack completely taking the place of the other person.I have never used multicam, and messed around with it, but because these various elements are not really things that were shot simultaneously on different cameras, I don't think that's the proper solution.To reiterate: I want to jump back and forth between the singer and the other person, while the singing continues uninterrupted even when the other person is on screen. And I would like to be able to jump back and forth several times during the duration of this sequence.One other thing: the performance of the singer runs somewhat longer than the clip of the person doing the other activities, and I would like the edited sequence to run as long as the singing.Thanks for reading this lengthy post and thanks in advance for your help.Steve

If I understood right, you just need to detach audio from video track by right-clicking the clip and selecting "Detach audio". 

Place singer in primary storyline. That is the spin of your story, as FCP was designed.  

Place the cutaway clip above it, mute its audio or detach it.  

Cut out what’s not needed on connected clip.