Two people can use the same Steam family's library at once. To run two separate sessions of the same shared game, however, the family needs two available copies. Installing one purchase on two PCs does not create a second licence.
There is a useful exception to check before buying again: Remote Play Together. For an eligible local-multiplayer game, it lets friends join one owner's hosted session. That is a different feature, not a second Steam Families copy.
Count copies, not installations
Consider this hypothetical household. You own an eligible puzzle game and a racing game. Your sibling wants the puzzle game while you want the racing game. Under Valve's Steam Families rules, family members can play different shared games at the same time.
Now you both want to launch separate sessions of the puzzle game. You can take turns or obtain the additional copy needed to run those sessions together.
If you want to play together inside the same local-co-op session, check the game's Remote Play Together support instead. Valve says only the host needs to own and install a supported game. This does not turn every online-multiplayer title into a one-copy game.
A practical check before arranging a session: confirm that the title supports the kind of multiplayer you want, supports Family Sharing, and has enough available copies. Those are separate questions.
DLC needs a look too. Valve warns that switching between copies with different DLC can affect whether a save works. Check which copy you are selecting if family members own different editions.
One household, separate accounts
Valve says Steam Families is intended for a household of up to six close family members. Everyone uses their own account; setting up sharing is not a reason to hand over a password or Steam Guard code.
The current FAQ describes separate saves and achievements for family members. Follow its Family Management instructions rather than an old beta-enrolment guide.
This arrangement lets people at home try eligible games while keeping their own progress. It does not transfer ownership. Steamworks documentation describes shared licences as temporary: access through a family is not the same as receiving a game as a gift.
Check the exceptions before buying
Not every title can be shared. Valve lists exclusions, including games with certain third-party account or subscription requirements and titles whose publishers opt out. Check the actual title, not just whether another game from the same studio works.
Valve has membership cooldowns, too. Read the current conditions before leaving a family or replacing a member. If an invitation is ineligible, use the explanation in Steam and its support guidance.
Before buying a second copy, ask what problem you are solving: is the game unavailable for sharing, or is an eligible copy simply in use? Only the second is the copy-count problem described here.
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