Why not shared memory?

[Raap]

Now that X2 cards are the new hotness, I've been wondering why they're not developing some way to let the two GPUs share the on-board memory. I can see why this isn't done on the Nvidia cards, which are just two cards slapped together, but with ATI's 2xGPUs on one card shouldn't it be relatively easy to get them both to access the same memory? I assume there's a good reason for this, but could someone explain it?

 

[LstBrunnenG]

That memory isn't going to be filled by just textures, each GPU needs space to do its own work without clobbering the other GPU's work.

It's like two students sitting at a table. They can share a textbook if they're doing the same assignment, but once in a while they'll need different pages, because they're not always going to be working on the same problem. Also, it's going to become a jumbled mess pretty fast if you try to make them use the same sheet of scratch paper.