So basically it’s a nuzlocke done by two people at the same time on the same game (or mirror game, like one plays X, the other plays Y). Nuzlocke rules such as one catch per route probably applies.
-How it works is that the one encounter on each route is linked for the two players. If player 1 fails to catch the Pokemon on that route, player 2 has to fail the catch or release or box the Pokémon even if he caught it.
-If both players manage to catch the Pokemon on that route (let’s say player one got a Zubat and player two got a Geodude) those two Pokemon are linked as partners.
If player one’s Zubat dies, player two must box his Geodude. If player one’s Zubat is boxed, player two’s Geodude must be boxed.
Hope this helped!
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