The short answer
Portal 2 offers a solo campaign and a separate campaign designed for two people. Co-op is not simply another person joining the solo story. Choose solo for independent discovery; choose co-op for a shared puzzle-solving session.
Choose solo when you want your own pace
A useful reason to start alone is control over the process: you can stop, reconsider an idea and discover a solution without another player moving ahead. This is an editorial recommendation about how to choose, not a claim that one route is required.
Choose co-op when you want to think aloud
Before starting, agree that both players get room to test ideas. Describe what you see and what you are trying to change. If one player already knows a solution, ask whether the other wants a hint before explaining it. That agreement matters more than treating the mode as a race.
What to check before arranging a session
Open the current Steam listing together and check the multiplayer features relevant to your setup. Confirm device and account requirements before buying. This guide does not establish crossplay compatibility with another platform and does not replace current setup instructions.
What to read next
If the goal is choosing a game for an evening together, compare the activity first: puzzles, a shared farm, or action. The Play together selection guide links each option to its game details. We do not rank unlike activities as though one score can settle the choice.