Both of mine work in unison in the theft department
Becky will immediately spot any ball on the beach/park, she'll hover hopefully, dancing on two legs round the owner and dog, looking highly excited. If they are daft enough to still throw it she and Jack will take off, outrunning (mostly) any dog alive to get there first.
If Becky gets the ball she will immediately either bring it back to the owner to throw again or, more likely, bring it to me or Hubbie for us to throw.
If Jack gets it tfirst hen its far more difficult. He's been taught 'Leave' better than Beccles but totally ignores any command for other toys in the offing

He'll prance about with the ball in his mouth, flaunting it in front of everyone, with Becky closely shadowing him.
If I call out 'Cheese', or open the zip on my dog bag, he'll drop it like a hot potato and hone in on a treat (walking stomach that boy

), whereupon Beccles will pounce on the dropped ball and fly back to present it for throwing.
At which point the owner is totally befuddled by this audacious pair of Artful Dodgers and is usually only too grateful to be presented with the ball back again (slightly soggy

) and beats a hasty retreat, whilst my two dance about, looking for their own balls in my bag.
One way or another, we get there in the end and haven't actually walked home with the wrong balls (unlike when other dogs nick theirs

)