I'd say that the aperture is too large for these shots. I understand that it is tempting to use the largest one the lens allows (or close to it) to try and get the narrowest DOF. However, it is generally too narrow for these shots.
I'd recommend getting a DOF calculator (if you have a smart phone, there are plenty of options) and using it until you really get a feel for these shots. I think you would surprise yourself when you plug in the numbers and find out what a more appropriate aperture would be. Remember that a person's face is generally a foot deep. So using an aperture that would allow for 6-12" of focus is what I would try to use.
Of course all rules are made to be broken, but this will help you get more of the faces in focus and make for a much more interesting photo.