If two players each control a Vorinclex, Monstrous Raider, do their effects cancel out or compound?

Vorinclex, Monstrous Raider has two replacement effects: one that doubles counters placed on permanents or players you control, and one that causes opponents to put half as many counters (rounded down) instead. When two players each control a Vorinclex, both sets of replacement effects are active simultaneously and they do not cancel each other out — they stack and apply together.

Replacement effects are governed by CR 614 and CR 616. When multiple replacement effects apply to the same event, the affected player (or controller of the affected permanent) chooses the order in which to apply them per CR 616.1. Both effects still apply; they don't 'see' each other and nullify themselves.

Consider what happens when Player A places a counter on their own permanent: Player A's Vorinclex doubles the counters. Player B's Vorinclex halves them (rounded down). The two effects compound — if you'd place 1 counter, Player A's effect makes it 2, then Player B's effect halves it to 1. The order chosen can matter for the final result.

Concrete example: Player A proliferates, adding 1 counter to Player B's planeswalker. Player B's Vorinclex doubles it to 2. Then Player A's Vorinclex halves it (rounded down) to 1. Alternatively, applying Player A's halving first gives 0 (rounded down from 0.5), then Player B's doubling gives 0. Player B, as the affected player's controller, chooses the order — so Player B picks the more favorable sequence (double then halve = 1 counter).

The key takeaway: the effects compound multiplicatively and do not cancel. The player whose permanent or loyalty counter total is being modified chooses the application order per CR 616.1, which can be strategically significant.