As with the PS4, the Xbox 720 looks certain to use AMD graphics hardware. VG247 reports that multiple sources have confirmed to it that the console will use two GPUs in tandem, it goes on to say that these won’t be used in the usual ‘CrossFire’ configuration seen in PCs. We’re not sure about all this as complex hardware architectures have been the failing of many consoles in the past, as a simple architecture is easier for developers to get to grips with quickly.

The next Xbox is to be as popular and last as long as the Xbox 360 – without the frankly appalling hardware failures which blighted its early days – it’s going to need some reliable and powerful components.
Rumours suggest that the console will contain a revision of AMD’s 7000 series graphics, which is based on its 28nm Graphics Core Next (GCN) Southern Islands tech.
Anonymous sources are being quoted on VG247 as saying that the graphics setup in the Xbox 720 will be “like two PCs taped together” which sounds like waffle to us. What does that even mean?
The same sources say that the two GPUs in the Xbox 720 “aren’t structured as they are in a normal dual PC set-up,” with each chip working separately to draw different items simultaneously.
Again, this sounds rather wooly to us, so let’s try and make some sense of it…
Perhaps it depends what the source is referring to as different ‘items’. The traditional usage of multi-GPU tech is Alternate Frame Rendering (AFR) where each GPU renders a frame in turn; if the source is saying that is not the case in the Xbox 720 then it will be a whole new usage of twin graphics chips.
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It’s possible this simultaneous rendering of different items could be referring to using the twin GPUs to display 3D outputs – where each chip is rendering a different angle of a scene to generate the 3D effect. This would then minimise the stress that 3D gaming puts on graphics chips.Still, if that secondary GPU is lying dormant during non-3D gaming it would be rather wasteful, so the GPUs must surely still be used concurrently to render the games.
It’s also possible the confusion here could be arising from the difference between AMD’s old GPU architecture, used in the Xbox 360, and the new Graphics Core Next technology. The old Very Long Instruction Word (VLIW) architecture of AMD’s last gen GPUs essentially batched up instructions before outputting them.

inforn=mation taken from:
http://www.techradar.com/news/gaming/consoles/xbox-720-release-date-news-and-rumours-937167