Can 2 people play a Steam game at the same time?
If 2 people are using the same steam account, no. A single steam player can only play one steam game at a time. If 2 people are using their own account, thus 2 different accounts, then yes. Each owns their own copy of the game.
How do I play a game with someone on Steam?
Make sure your friend is logged in to Steam from a supported device. Open your Steam Overlay in-game (Shift+Tab). From your friends list, right-click your friend and select 'Remote Play Together'. Once your friend accepts the invite, they will be in-game with you.
Can you play on the same Steam account at the same time?
Steam does not support multiple players using one Steam account simultaneously - games associated with a Steam account are licensed for the sole use of the account holder.