From Tech Report's preview:
We do know, however, that the two GPUs on the card don't share memory. The board has eight Hynix GDDR5 memory chips per graphics processor, four on the front and another four around back. Those chips are 1Gb each, so each GPU has a total of one gigabyte of memory to call its own. Cumulatively, 4870 X2's effective memory size is still 1GB, since data must be replicated into each GPU's memory space.
http://www.techreport.com/articles.x/15105

It's counted as 1 GB because the data needs to be replicated on both cores. Hence, only 1GB amount of data can be used at an instance.

UPDATE: Our initial publication of this article indicated a 2x 512MB framebuffer for a total of 1GB on board. We have since learned that the R700 we tested has 2GB of RAM total for 2x 1GB framebuffers. This has affected some of our analysis and conclusion. We do apologize for any confusion this may have caused.

what is this micro stuttering that its called?

2gb