Choose which VPN you prefer and uninstall the other.
I don't run Avast One but do run Avast Premium Security, which does have Avast VPN as a feature. As an Avast user, not as an Avast team member, I cannot directly help you with your disconnection issues, but, if need be, see if one can come in if you request.
Why run/have installed two VPN's at the same time?
If you choose, you can modify what features Avast One has (Avast One VPN is one of these) and choose Custom Install and deselect that feature even if it is free. Restart your system when done.