If two players both want to activate abilities at the same time who gets priority first?

Priority determines who gets to act first in Magic. According to CR 117.3a, the active player — the player whose turn it currently is — receives priority first at the beginning of each step and phase. This means if both players want to activate an ability at the same time, the active player gets to go first.

After the active player takes an action (like activating an ability) or passes priority, the next player in turn order receives priority (CR 117.3b). Players must pass priority in succession without anyone taking an action before a spell or ability actually resolves (CR 117.4).

If both players pass priority in succession with nothing on the stack, the current step or phase ends. If abilities are on the stack, the topmost one resolves, and then the active player receives priority again (CR 117.3c).

Example: It is your turn and you both want to activate abilities at the same time. You, as the active player, activate your ability first, putting it on the stack. Your opponent then receives priority and may activate their ability in response, which goes on top of yours. Abilities then resolve last-in, first-out.

So there is no true 'simultaneous' action in Magic — the active player simply always has the first opportunity to act, and players take turns holding priority around the table in turn order (CR 101.4, 117.1).