I didnt see it listed so heres how u get it
2 Players Mode
Here and there it exist secret 2-Players icons (there are two heads on it).
Go near it, and it will ask you to connect an extra controller in the second
controller port. It exist Rampage and Free Roam-icons, and you can play some
minigames together, like pool. In Rampage you and your friend usually just
kill some peds in a limited time period. In Free Roam, you walk around and
just have fun.
But the 2 Player Mode is limited to no missions at all and you must both stay
on the same screen.
You often find a 2P-icon near your girlfriends houses when you have got a
specific % with that girl.
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*Free Roam Los Santos*
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In NorthEast Idlewood. About 2 Blocks North of the Tattooist Place,
just south of the highway, there is an open building with fences around it,
and a sort of archway door. Go in there and there it is, a red icon with two
people on it. You don't have to have done anything to get it.