With a white item, it'd only be a 4% gain, and not an 8%. The crit chance here would then be 7%, not 14%, if this was indeed done additively. If it was done multiplicatively, it would be 6.5% or somewhere thereabouts. It would be extremely hard to test if this is done additively or multiplicatively on white gear.

Even on blue gear at a 6% chance, additvely it'd be a 10.5% addition to crit, multiplicatively it would be 7.5% chance to crit. 3% may be easier to test than .5% (if .5% even matters, and isn't rounded out by the game).