If multiple players scry at once, each of those players looks at the top cards of their library at the same time. Those players decide in APNAP order (see rule 101.4) where to put those cards, then those cards move at the same time.
July 3, 2020 · Core Set 2021
rule 701.22c not present in this release
June 8, 2018 · Battlebond
Each clashing player reveals the top card of their library at the same time. Then those players decide in APNAP order (see rule 101.4) where to put those cards, then those cards move at the same time.
April 27, 2018 · Dominaria
A player wins a clash if that player revealed a card with a higher converted mana cost than all other cards revealed in that clash.
September 29, 2017 · Ixalan
A player may planeswalk as the result of the “planeswalking ability” (see rule 901.8), because the owner of a face-up plane card or phenomenon card leaves the game (see rule 901.10), or because a phenomenon’s triggered ability leaves the stack (see rule 704.5w). Abilities may also instruct a player to planeswalk.
September 30, 2016 · Kaladesh
A player may planeswalk as the result of the “planeswalking ability” (see rule 901.8), because the owner of a face-up plane card or phenomenon card leaves the game (see rule 901.10), or because a phenomenon’s triggered ability leaves the stack (see rule 704.5x). Abilities may also instruct a player to planeswalk.
February 1, 2014 · Born of the Gods
Schemes may only be set in motion one at a time. If a player is instructed to set multiple schemes in motion, that player sets a scheme in motion that many times.