Yes, the x2 isn't anything special technology-wise. It's just Crossfire on a single board.
It has the same limitations and features as cross-card Crossfire, and works the same way (each thing written to memory must be duplicated in each frame buffer).
Same goes for NVIDIA's SLI. If you see products which are built on SLI or Crossfire (2 GPUs on 1 board), remember that no matter what memory amount they advertise, you're only really getting to use half of that.
For instance, the 3870x2 and 9800GX2 were both marketed as having 1GB of DDR3 memory, when in fact they had 2 x 512MB (512MB is duplicated, so you only really get half the advertised amount for "using").