Yes pal, only one app open at a time.
I’m thinking it could be a phone issue at this point, though I downloaded an app to test my ant+ and it says it OK. Lol.
As far as my problem with erg power being wonky, I’m thinking it might be an issue with my dongle/extension cable/5 year old laptop.
I think tonight when I try trainerroad with my wife’s phone (an s10) ill find out conclusively if it’s my trainer that’s broke.
If you are connecting to The Tacx app and it is not working properly to control the power, then it’s a problem with the trainer. Contact Tacx. Hopefully they will help you.
Frustrating, I understand. Tacx will sort it out I’m sure.
In the meantime, if you have a regular turbo trainer, a speed sensor and a cadence meter, you can use virtual power in TR or Zpower in Zwift and still do the interval workouts effectively. These tools reverse calculate your power from speed (and trainer model) and work great.
Good hunting.
GREAT SUCCESS!!
Just completed Donner using my wife’s phone (an older model of my own) and it worked flawlessly from start to finish.
The good news is my trainer and thus my sanity is ok, the bad news is my phone/trainerroad connection needs dealt with.
I’ll experiment more tomorrow and report back!
Happy days!
phew 
Just to put an end to this and hopefully help someone in the future who Google searches the same issue, this is 100% a problem with my Samsung galaxy s9 and it"s ant+.
I’ve tried factory resetting the phone, updating all software on android and turbo trainer and its still not working. I even downloaded the ant tester app and according to it the ant+ on my phone is working OK which it obviously is not.
When I connected zwift + trainerroad using laptop and phone (like I have done in the past with no issues - no need for anyone to talk me through the correct process) it completely buggered something in the phone.
I have since tried trainerroad and zwift (though not together) several times on my old laptop, a galaxy s8 and a galaxy s7 and all are running perfectly, with maybe the laptop just a bit behind in terms of adjusting erg mode like a more modern device would.
Hope this helps!