We have 2018 dual motor X, rear Motor just failed. Car let us continue but as soon as we stopped, car was disabled. You can not drive, even though we have dual motor. We weren’t stranded but now car has to be towed to be fixed. So, you can not drive if one motor still functioning. Not sure why Tesla told us that. Must have meant you can continue to drive when it fails but that is it.
It really depends on the reason for the failure. In general, it will be rare for a Model X or S to be able to drive on one motor as most failures will cause the car to think there are multiple system errors and just stop for safety.
It doesn't help that Elon tweeted this in 2018: https://twitter.com/elonmusk/status/998088849636016128?s=20 Ingineer in 2015 posted this about dual motor S failures: P85D Front Motor Died, which is a way that a rear motor failure could make the car unable to move.
If I have a motor fail, I will remember not to stop driving until I get someplace where it's convenient and safe to leave the car as it probably won't allow me to start driving again.