in the Hardware tab, you can define which GPUs are dedicated to encoding/playback.
If you have a single GPU, you’ll have to set it to "Playback and Encoding"
If you have two GPUs and want to throw the most power at encoding, you may be tempted to set both to Encoding Only, but at least part of one GPU has to be dedicated to playback. So, you can have
A: Playback Only
B: Encoding Only
A: Playback and Encoding
B: Encoding Only
but you can’t have
A: Encoding Only
B: Encoding Only
If you have it set up like:
A: Playback and Encoding
B: Encoding Only
You will basically be wringing all the power available out of it for encoding. It will only steal GPU away during actual active playback. You can even reduce this to give encoding priority by using the sliders at the bottom of the RENDER tab.