Are you talking about the M1 Extreme? This is extremely expensive, and I suspect the dual die GPU only works because of Metal.

TBH I prefer AMD's 7900 series approach of pushing the memory controller + cache out onto little memory controller dies. This allows for a monolithic GPU with a really wide bus, without all that wasted space on the controller/cache/pins. And those dies don't care about being split up since the memory access is interleaved anyway.