Mantle was also capable of using the ram in this way when crossfired but no-one ever used it so I wouldn't go holding my breath for them to start using it now. I'm sure it will eventually become common place but it's unlikely there will be anything more than a gradual rise in it's use by games developers meaning most of us will be on the next gen of cards before it is worth making it one of the considerations when buying a dual card set up.
It's pie in the sky like Lucid Logix et al. Any implementation we do see won't be like AFR where FPS scaling approaches 100%, it will be mundane like the SFR on CiV: BE.