Exclusively using the dGPU is supported by the hardware, but it isn’t exposed as a BIOS option since it would result in dramatically worse battery life, and currently cannot be configured by the Linux drivers. See this thread for more information: [RESPONDED] MUX switch toggle in BIOS?
It is possible to exclusively use the iGPU for putting the final rendered image on the screen, but this will likely result in higher latency. The Arch wiki has a brief section on this configuration: PRIME - ArchWiki