Physically stacking? No, for a couple of reasons:
- It would require a great deal of manual wiring, given that RDRAM is controlled in a daisy chain, where you would have have to wire SIN / SOUT of each chip... it would end up not being a 1:1 connection.
- RDRAM runs at 500MHz, which makes impedance-matching wildly important. It is so sensitive that many of us here have had trouble getting a stable Akira Method / other form of RAM termination on our trims. Manually wiring RDRAM together / physically stacking will lead to mismatched impedances and lots of crosstalk, which will result in the system not working.
- The RAM is the hottest part of an N64, so stacking them would end up overheating the chips, even if you slapped a heatsink on top of the top two.
If you really wanted to, you could create a custom PCB that mimics some of the
3rd party expansion paks that use 2 2mb RDRAM chips, then try something like a RAM bend? I'm not even fully sure that would work, but it's a possibility. Definitely not compact, that's for sure.