Ideal would be to map /downloads the same on both containers.
For example:
Sonarr: /downloads/tv <-> /mnt/user/downloads/tv/
NzbGet: /downloads <-> /mnt/user/downloads/
or
Sonarr: /downloads <-> /mnt/user/downloads/
NzbGet: /downloads <-> /mnt/user/downloads/
Alternatively you can setup a Remote Path Mapping so Sonarr is aware of the different mapping.