You use two or more computers at the same time?

This is directly from Steam's support page on the topic:

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.

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).

It's literally worked that way since 2003.