Under the current rules (as of writing this blog post) both of them would simply die as a state-based action due to the planeswalker uniqueness rule, so nothing special happens
This already makes Stock Up the best option for card advantage/selection in formats like Standard and Pioneer, but when you consider the formats that include free interaction the value gets ramped up to 11
Paragon of Eternal Wilds, Hunter's Ambush, and Sylvan Anthem each take advantage of your many green creatures
Spectacular Spider-Man Cheap flash creatures always offer great versatility since you can hold them up alongside interaction
Julius Jumblemorph Ego Erasure Feisty Spikeling Firdoch Core Fire-Belly Changeling Flock Impostor Formless Genesis Game-Trail Changeling Gangly Stompling Ghostly Changeling Gladewalker Ritualist Graveshifter Guardian Gladewalker Impostor of the Sixth Pride Irregular Cohort Littjara Littjara Glade-Warden Littjara Kinseekers Masked Vandal Maskwood Nexus Mirror Entity Mischievous Sneakling Mistform Ultimus Mistwalker Moonglove Changeling Moritte of the Frost Morophon, the Boundless Mothdust Changeling Mutable Explorer Nameless Inversion Omni-Changeling Omo, Queen of Vesuva Overlord of the Hauntwoods Personify Prideful Feastling Realmwalker Rooftop Percher Runed Stalactite Shapesharer Shields of Velis Vel Sizzling Changeling Skeletal Changeling Springleaf Parade Stalactite Dagger Taurean Mauler Three Tree Mascot Turtleshell Changeling Universal Automaton Unsettled Mariner Valiant Changeling
Some, like Crackling Cyclops, grow stronger with each noncreature spell you cast, while others, like Balmor, Battlemage Captain, buff every creature you control
