Why do 2 people yawn while talking to each other on the telephone?

Sometimes while I am talking to my wife on the phone, she'll yawn and lo and behold, before the next 10 seconds, I too will yawn. Got an answer? Can you help the brother out?

This is an interesting question. I studied both science and education degrees at uni and it was mentioned in both teaching and animal behaviour!! This is because when one person yawns it begins a fixed action pattern (FAP) where it almost inadvertently leads to other people present yawning too. This process can be started by being around someone who yawns, hearing someone yawn, reading about yawning or even seeing a picture of yawning. It is an automatic sequence, once we start that sequence, we find it very difficult to stop. There are also examples of fixed action patterns in lots of animals.

Agreed its interesting!
Only thing I can think to compare it to is a defensive innate response. Might be on the scale of vomiting - if someone near you vomits you feel the need to vomit. Its a safety feature from nomadic days - if someone eating the same mammoth as you vomited, there may be something wrong with the meat, so you vomit as a protective device.
Yawning occurs to stretch the alveoli in the lungs and increase their lubrication, thus increasing their efficiency for gas exchange. Might just be that when one person yawns, the other, as a protection against low oxygen content in the air or need for potential energy follow suit.
Note this is just speculation.