Does menace require both blockers to be declared at the same time?

Menace requires that a creature with menace can only be blocked by two or more creatures. This restriction is evaluated during the declare blockers step, when all blocking assignments are made simultaneously. A single blocker cannot be declared and then a second added later — all blockers for any given attacker are chosen at the same time (CR 509.1).

When the defending player declares blockers, they announce all their blocking creatures and which attacker each is blocking in one action. If a player tries to block a menace creature with only one blocker, that blocking declaration is illegal and must be corrected before the game proceeds (CR 509.1b, 509.2).

Menace itself is defined in CR 702.110a: 'A creature with menace can't be blocked except by two or more creatures.' This is a restriction on the legality of the block, checked at the moment blockers are declared. It does not create a two-step or sequential process.

Example: Your opponent attacks with a 2/2 with menace. You have a 1/1 and a 2/2 available to block. You must declare both the 1/1 and the 2/2 as blockers for that creature in the same declare blockers step. You cannot legally declare just the 1/1 as a blocker.

Note: Some effects can add creatures as blockers after the declare blockers step (e.g., certain triggered abilities), but even then, menace's requirement of two or more blockers must already have been satisfied when blockers were legally declared — you cannot use such an effect to 'fix' an otherwise illegal single-blocker declaration.