Clamshell mode for GDDR5 does not do what you say it does. Clamshell mode allows for a single memory controller to access 2 memory chips, it does not allow for 2 memory controllers to access a single chip. So no shared memory through clamshell mode because a single memory chip can only cope with 1 controller and not 2, which is need for your shared memory approach.
Basically clamshell mode allows you to use the same 512Mbit chip for both a 512MB 256-bit card using 8 chips and a 1GB 256-bit card using 16 chips. That's also what spursindonesia is referring to.
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