Looking at the Orbot app, looks like maybe they’ve make some changes. At one point, it was unusable for me. May give it another try. 

How does this work if I’m using Mullvad VPN? Can I use both at the same time? If not, which would be better to use? 

 Not if you are on iPhone. The iPhone will only allow one at a time. If you are on Mullvad, Orbot will kick you off. 

Depends on what you are doing