We are hiring a car, I will still be 20 so Joanne is the main driver.
I'm a little curious/frightened of what it sounds like you are planning here.

If you are under 21, driving a rental car will be pretty much off limits. I don't know of any rental agency that will allow an extra driver under age 21, certainly not without some huge extra fees.

If you are not on the rental contract and you drive the car, you will essentially be driving a stolen car. That will create all sorts of problems. At the very least any insurance would almost certainly be void, so if you were to get into an accident, you then would become personally responsible for any and all damages you'd cause not only to the car, but to anyone or anything else affected by a crash.