Tim, probably little to no real benefit in trying to force the Nvidia dGPU as being prime for PD on a laptop. It could even be a performance detriment depending on laptop MB architecture. I'm not that familiar experience wise with AMD's embedded graphics processor as much as Intel, but I'd think overall characteristics would be about the same.
Tasks really don't spread amongst GPU's, however, each one can do a specific task depending on settings. One can decode, one can encode, so on. Both won't perform a encode task simultaneously. When timeline content is of high quality and complexity, say high bitrate H.265 content which requires decode performance, it can be advantageous produce wise to do decoding on one GPU and encoding on the other. However, if one is dealing with lower bitrate and lower fidelity compression like H.264, no significant gains in splitting of these two tasks on different GPU's.
Your AMD is probably always prime GPU for PD in that laptop when both GPU’s are enabled regardless of settings, so it will do the timeline decoding. Render preview encoding is always 100% CPU, decoding could be CPU or prime GPU depending on PD settings. You also have the option of using Nvidia NVENC for encoding during produce if desired. As such, the Nvidia will never perform timeline decoding, just encoding. Decoding would be CPU or embedded GPU.
Jeff