Any software to multiclient simultaneously?

[-Setro-]

Say I want to run 2 clients of a game, and control both clients simultaneously. How can I do this?

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[Tayasaurus]

Sandboxie

http://www.sandboxie.com/

[-Setro-]

Sandboxie

http://www.sandboxie.com/

This does not let you control them simultaneously, it only lets you run two copies.

[Headcrab Zombie]

Depends on what game.
Some you already can
Some you need to make modifications to the game
Some you'll have to run VMs


Check resources (google, forums) specific to the game