Having two dogs, or more, is not always fair or easy. I, like most people, experience some level of guilt when I reward one dog and not the other. I have to make a real effort to not give in to what I think I hear from them " but it's not fair! He got a cookie why can't I?" 
I bring this up because I am working on fast responses to recalls. I am only rewarding the one who responds within a "one good dog" count. 
I am watching for a head snap as soon as I give the cue. The dog who decides to finish his sniff or continue walking loses out.
It's amazing how quickly the slower dog will begin to pay attention and respond faster once he realizes there is no payout for him.
It's not easy ignoring the big round eyes, the head tilt and the expectant expression. It has been paying off though because I am getting better responses :)
I make sure at some point during our walk, the slower one does get rewarded for something.....I'm not that heartless :)