So we all know and love the young dragon, Spike, from Equestria who was an orphaned egg before he was hatched by Twilight during her entrance exam. Then he was adopted into her family, learned a few magic tricks with his firepowers from Princess Celestia (at least according to Lauren Faust), worked alongside Twilight as her best friend and partner, went off on some death defying adventures, saving the Crystal Empire, played a major role in Dragon Lord Ember and King Thorax's coronations, grew a pair of wings, etc.

Meanwhile, in Equestria Girls, we have a dog counterpart of Spike who sadly isn't given enough development in comparison to his dragon counterpart. Still some unanswered questions like where did he come from? How did he came to be Twilight's pet? Did the Equestrian Magic do more than give him the power of speech? Like can he see colors, eat chocolate, or do stuff that a regular dog can can't do?

But character differences aside, for all we know from the Equestria Girls specials on YouTube, or TV, whichever comes first, both Spikes have never met each other before, unlike Princess Twilight Sparkle and Sci-Fi Twilight, who've both met on a few random occasions. Once in the Friendship Games and a few times in that Spring Break special. Nevertheless, that never stopped us from making fan fictions where both Spikes met and worked together.

I remember once reading a fanfic where Dragon Spike wanted to take a break from his job in Equestria, while Dog Spike wanted to see what life is like as a fire breathing dragon. So they both performed the switcheroo routine, which lead into some troubles. Does anyone the title of the fanfic? Update: The title of the story is The Tale of Two Spikes.

I don't know the title of the story you're looking for, but I agree with you on the account of dog Spike. Was he a wee puppy when Twilight adopted him, and I'm sure we can tell she adopted him because she was lonely since all of crystal Prep hated her for biased reasons. And if the Equestria magic did more than give him the ability to speak, can he perceive all colors, eat certain foods dogs aren't meant to be given, or anything else. And the fact he's so far had no involvement in any of the latest Holiday shorts has me concerned.