We've mentioned the X2 version of the 5870 a couple of times already. Moments ago some pictures of 'Hemlock' actually surfaced.
The card indeed is slightly longer than the Single GPU based 5870,but seems to have better ventilation. It draws its power from one 6+2 pin and one 6 pin PCI-E power connectors.
To get a rough idea on how the 5870 X2 card will perform, have a look at this article.
Well, go have a look .. click on the thumbnails: