Steam does not support multiple players using one Steam account simultaneously. If a game is associated with a Steam account then it is licensed for the sole use of the account holder.

If multiple users attempt to log in and play simultaneously with one Steam account, the user who logged in first will eventually receive an "Invalid Steam UserID Ticket" error message and be forced to log back in to continue playing. If the first user logs back in, the second user will receive this error message. Effectively, a single Steam account cannot be used from two locations simultaneously.