Not multiplexed on boot — multiplexed at run time. Build a chip that speaks the GDDR protocol to the host GPU and has 2-4 GDDR channels coming out the other end and aggregates the attached memory at the cost of an extra chip, some latency, some power, and an extra chip. As far as the GPU is concerned, it’s an extra large GDDR chip, and it would allow a GPU vendor to squeeze in more RAM without adding more pins to the GPU or needing to route more memory channels directly to it.

(Compare to something like Apple’s designs or “Project Digits”. Current- and next-gen GPUs have considerably higher memory bandwidth but considerably less memory capacity. Mostly my point is that I think Nvidia or AMD could make a desktop-style GPU with 2-4x the RAM, somewhat worse latency, but otherwise equivalent performance without needing Samsung or another vendor to build higher capacity GDDR chips than currently exist.)