It's not like going from a small cake tin to a big one, where you need to add extra time because the heat needs to penetrate further. You're not adding any additional volume to the chickens so theoretically (since the chickens won't 'know' whether they're in two different ovens or a single one) you should cook them for the SAME time.
However, in practice, when increasing the number of items in an oven (but keeping those individual items the same size) you usually need to add a LITTLE extra time. (I'd guess at about 10 to 15 minutes in your case).
Irrespective of what is says on the packaging (or I've suggested here) you should always check that a chicken is properly cooked, as per these instructions:
"Check the juices run clear by pulling the leg away from the body and skewering between it - juices should flow out and if any pinkness remains, cook the bird for a further 10-15 mins. The fibres of the flesh should also be opaque and cooked, and no traces of pinkness or blood should remain".
Source:
http://www.bbcgoodfood.com/videos/techniques/how-test-joint-cooked-chicken