Does menace require both blockers to be declared before combat damage is assigned?

Menace is a static ability that restricts how a creature can be legally blocked. According to CR 702.111b, a creature with menace can't be blocked except by two or more creatures. This restriction is enforced entirely during the declare blockers step (CR 509.1), when the defending player chooses which creatures block and how many.

If the defending player cannot or does not assign at least two creatures to block a menace creature, that creature is simply unblocked — it doesn't receive partial blocking. There is no mechanism for a single blocker to 'join' later in the combat phase; all blockers must be declared simultaneously during the declare blockers step (CR 509.1a).

Combat damage assignment (CR 510) happens in a completely separate, later step. By the time combat damage is assigned, the question of whether the attacking creature is blocked or unblocked is already settled. So yes, both (or more) blockers must be declared before combat damage is ever assigned.

Once two blockers are legally declared, the attacking player assigns combat damage among them in any order they choose, but must assign lethal damage to each in sequence before moving to the next (CR 510.1c).

Example: Your 3/3 with menace attacks. Your opponent controls a 1/1 and a 2/2. During declare blockers, they assign both creatures to block your 3/3 — this is legal. Combat damage is then assigned: you could assign 1 damage to the 1/1 (lethal) and 2 damage to the 2/2 (lethal), killing both. If your opponent had only one creature to block with, they couldn't block at all, and your menace creature would deal its 3 damage directly to them.