How would I go about training my two dogs to blow air out of their mouths?

I have two hairless dogs and they both really enjoy having raspberries blown on their tummies so I thought "Wouldn't it be cool if I could train them to do this to eachother?" and yes, it would.

Now I realise there are some physical problems here like the lack of lips and the general issue of the shape of the mouth/snout but I don't think that would be a big problem, if they close their mouths and only blew gently it could definitely still work.

I can just imagine them blowing raspberries on eachother happily.

Ideas?