Does menace require both blockers to be declared simultaneously or can a second blocker be added after the first?

Menace requires that a creature be blocked by two or more creatures, but this condition is checked during the declare blockers step, not incrementally. All blocking creatures must be declared at the same time, in a single action by the defending player (CR 509.1).

Under CR 509.1, the defending player declares all their blocking creatures simultaneously. There is no game rule that allows a player to 'add' a blocker after the declare blockers step has concluded. Once blockers are declared and the step moves forward, the blocking assignment is locked in (barring specific card effects).

Menace itself (CR 702.111b) states that the creature can't be blocked except by two or more creatures. This restriction is evaluated at the time blockers are declared. If the defending player only declares one blocker against a menace creature, that declaration is illegal and must be corrected before the declare blockers step ends — they cannot patch it by adding a second blocker afterward.

There are card effects that can cause creatures to become blockers outside of the declare blockers step (e.g., cards that say 'tap this creature to block'), but these are exceptional cases defined by the card text itself, not a general rule. Without such an effect, no second blocker can be sneaked in later.

Example: Your opponent attacks with a 2/2 creature with menace. During declare blockers, you only have two 1/1 creatures available. You must assign both 1/1s as blockers at the same time in the declare blockers step. You cannot declare just one 1/1 as a blocker, wait, and then add the second one — that would be an illegal block from the start.