Adding the 2nd RAM stick and enabling dual-channel will not fix its only 8 GB of VRAM, as the GPU part is not upgradable. Just get a PC and install SteamOS. One might call the 8 GB of VRAM VALVE's strategic mistake (look, even the big reviewers on YouTube complain about the only 8 GB of VRAM and they have millions). The 8 GB of VRAM issue started like 4-5 years ago. Here is a more recent one about only 8 GB of VRAM: youtube.com/watch?v=ric7yb1vaoa ("gaming laptops are in trouble - vram testing w/ @hardwareunboxed"). Indeed, the recent memory price increase doesn't help this issue.
Despite its lofty price, Steam Machine performance is not as optimized as expected. To address high DDR5 RAM costs, Valve installed one 16 GB module. New benchmarks confirm that lacking dual-channel memory, apps and games produce significantly lower scores and frame rates.