Forgive my resurrecting this zombie thread, but it is one of the ones I found when recently investigating the situation of two cars in one Tesla account. In my case I'll only ever need to give one of them - the new one - the 'extra' Intelligent hours (the other does significantly lower mileage).
The Octopus app system has been playing up recently (would only work with iOS phones) but it was in any case refusing to work with our 'main' account now it has two cars in it.
The workaround (as alluded to in post #2 above) is to create another (new) Tesla account - this is easier if you can use a spare phone (because your main phone will 'forget' its ability to be the car key if you sign out of the app).
Invite that new account as a driver of the car which you would like to work with IO. This is done from 'Manage drivers' at the bottom of the car's screen in your 'main' Tesla account.
Once the car has been added to the new account you have a Tesla account with only one car in it. Sign up (or re-connect) the IO App to that account instead, the test charge will run and all will be perfect. Naturally your 'main' Tesla app still has access to both cars.
Hopefully this spells out the steps in way which will be useful for someone...
Cheers, Dave