The beginning command can be, Go find mommy. Daddy says this and releases the hold on the collar. Mommy pulls on the leash to help the dog find her. Mommy gives the dog lots of praise and a reward. Now you switch.
Daddy holds the end of the leash and mommy holds onto the collar.
Mommy says, Go find daddy and releases the collar. Daddy pulls on the end of the leash to guide the dog to him, gives him praise and a reward. You can keep practicing this until you no longer need the leash. Now, you can separate yourselves even further apart and eventually go into separate rooms and your dog will go find the other person. Eventually you can give your dog something to hold in his mouth to give to other person.