Hello I’m brand new using Audacituy and in lockdown of course I’m playing cello duets and trios with myself. The only problem is the other cellist’s timing is rubbish, 
they tend to speed up when the music gets loud. So I’ve made a rhythm track so they keep themselves in order 
. But I’m not sure how to start it and record myself at the same time. I’ve checked that I’ve got the overdubbing on, so that’s good, but I just don’t know how to start both at the same time.
Help please.
The job of overdubbing is to play a rhythm, backing or accompaniment track to your headphones while you’re performing to a new recording. There’s a setup you have to go through to make sure everything lines up. It’s not push-a-button automatic.
In one scenario, you can Generate > Rhythm, Metronome, or Click Track and play that as a reference of timing. There are different ways to generate those because the click patterns were getting too complicated (3:2, syncopation) and actual simple metronome was getting hard to find.
https://manual.audacityteam.org/man/tutorial_recording_multi_track_overdubs.html
Koz
There is another oddity. You can’t listen to your computer and hear yourself perform at the same time. The computer delays make your own live work sound out of time or with an echo. That’s computer latency.
What you can do is listen to your microphone, sound mixer or USB interface if it has that provision.
If it doesn’t, then you’re stuck with hearing everything but your own live performance in the headphones. That’s why this singer is only using one headphone muff.
Koz