--While driving around in the area that is open by default in this game, I came upon an icon. It happened to be two little red people spinning in circles, signaling this is where the two player mode in GTA: San Andreas is. I brought a friend over so he could play with me, as it requires someone on the 2nd controller to use. The first thing that is shown is two people next to each other, telling you to roam free around the city. Then, the second player gets the chance to choose which character he wants to be. These range from cowgirls to old bums to police officers. They all behave the same, just different costumes really.
--You then get returned into the game world and it adds a second character info area in the top right of the screen below yours. You share the same inventory, but have separate ammo amounts. There is no split screen; you just have one camera that accommodates to how far apart you are from each other. To keep things manageable there is a set distance apart the two of you can be. Two player mode basically runs as a mission. When one player dies, the mission is over. The same goes for being arrested. One really fun and cool feature of this mode is that both characters can get into a vehicle together. One person obviously drives, and the other gets an aiming reticule that shows up and you can shoot at will.
--There are probably quite a few more things that you can do, but I have not had the time to explore these to detail quite yet.
Enabling Multi-player, go to the town where the county PD station, find the big store, next to it you will see a little red icon with 2 players on it, plug in your second controller, press left or right and choose player 2's costume, cop, lady, scruffy old man etc
also in Flintwood, you can do this but I cannot find where. In this mode you can go swimming together, in the same vehicle or seperate bikes, you cannot enter buildings, although this mode has a few restrictions, this game mode is really good.