You can use GPU for:
1) decoding
2) SVP calculating
3) picture rendereing
You can check which GPU is used and change it.
Decoding. Just open LAV decoder options and choose Hadware Device to use: MPC-HC menu View - Oprions... - Internal filters - Video decoder
SVP. Use SVP context menu to select: Application settings - GPU acceleration
Rendering. Used GPU physically connected to your display. You can reconnect