Given the problems SLI and crossfire had with more than one gpu, but in the same system, I doubt even valve would be able to combine different devices in a very meaningful way. The best I can imagine would be a pc rendering a game which is streamed to a steam deck, which in turn throws all its resources on FSR-like upscaling. Or, in case of a VR device that it handles interframe generation and reprojection locally on a stream rendered by a different device.