Poor NVENC Quality / Potentially not making use of multiple encoders?

[Kally]

For constant bitrate I'm able to get a significantly better looking image out of OBS in real-time, whereas when using Avidemux it is below real-time and the quality is worse.  I'm using an RTX 5090, which has 3 NVENC chips, and I believe it is only using one.  Is this true, and if so are there any plans to change that?