Super Smash Bros. Brawl Adventure Mode (Subspace Emissary) Is Single-Player Only

No. The Adventure Mode (Subspace Emissary) in Super Smash Bros. Brawl is strictly single-player. You cannot play it cooperatively with a second player.

Multiplayer Alternatives in Brawl

• Versus Mode: 1v1 or team battles with custom rules.
• Classic Mode: Solo gauntlet; friends can take turns.
• All-Star Mode: Survival challenge; turn-based co-op possible.
• Stadium Mode: Some mini-games (Home-Run Contest, Target Smash) support 2 players.
• Subspace Emissary (Partial): A few forced segments let you control two characters simultaneously, but it's not true drop-in co-op.

For a full co-op campaign experience, later entries like Super Smash Bros. Ultimate's "World of Light" still remain single-player only—co-op is limited to Spirit Board battles and Versus modes.