According to "How Volvo Created the Jean-Claude Van Damme ‘Epic Split’ Video":
Van Damme, 53, was hooked to safety lines that aren’t visible in the film, and each foot is on a small platform that was built on the trucks’ side mirrors.
Van Damme’s feet aren’t secured to the mirrors, video director Andreas Nilsson said, “but we had him rigged so that if he would fall off he wouldn’t die obviously. We didn’t want to be responsible for killing the Muscles from Brussels.”
I checked, to the best of my abilities, for discontinuities or any unexplained shadows, reflections, backing-up lights not being on, light/shadow on JCVD's face, etc. and couldn't find any. It doesn't help that the sun light is from behind. However, perhaps there is a technique to identify some CGI being used in certain areas of the frame. (Perhaps covering up some cables, although they could also be hidden behind him or in his pants.)
(One remaining doubt: the relative height of the trucks, seemingly changing around 0:36, which might be explained by the right truck moving slightly faster? Or is this explained by the moving away of the left truck, at that time?)
From the movement of JCVD's pants bottoms as they fall from his boots (from 0:41) it can also be deduced that it is very unlikely that the film was played backwards (in case you were wondering).
Note that it is relatively difficult for a combination truck to drive backwards in an extremely accurate fashion. (Try parking backwards with something attached to the back of your car, let alone driving in this fashion.) However, the combination truck on the right "only" has to keep a straight line (possibly helped by the Volvo system that's being advertised), whereas the left truck is rigid, and therefore much easier to manoeuvre. This is the only truck, in the video, that moves away to accommodate the split.
There's (now) also a press release by Volvo Trucks.
I almost forgot: Unless JCVD's whole career was faked, there's isn't much reason to doubt his "splitting abilities".