Huh, not sure how to train it, but Niko knows Mommy and Daddy. DH will ask him "Where's Mommy?" and if Niko runs to me, I praise him and give him treats/petting. I can say to Niko, "Go get Daddy!" and he will run to DH. We didn't work on training it, but I may have called Niko after DH asked him where I was. Eventually, I didn't have to call him anymore.
We tried to do the same thing with Rosa, but I'm not sure if she gets it. She has a mind of her own, and if she doesn't want to find the other person, she just won't.
Niko: American Showline GSD 5 years old
Rosa: American Muppet Dog (GSD/Border Collie mix) 5 years old