The image above shows a CT Setup you could use on Dust2. Two players hold B, and two players hold Long. Player 3 sits in Mid and watches Catwalk. As soon as a Terrorist moves up Catwalk, or if Catwalk is smoked, or he is flashed, then he calls for one of the players at Long to rotate back to A Site. At this stage you could then probably leave only one player at B, and have the other rotate to CT Spawn, so he will be quickly available if they rush A. Obviously if the bomb is spotted at A then everyone should be rotating towards the site. The benefit of playing two long and two B is that if they do a fast rush at either of these places then you have a much greater chance of shutting them down. The player in CT Spawn can also help out if they rush Mid, and the player in Mid can also help if they rush B.
This is how you should be setup if you are expecting them to do an A split.
If they go B then one player at Long should push through doors and get into tunnels, the other should move through CT Spawn.
If the T’s attack Mid then you can have one player at B watch from Window to help out, also one player from Long can move to CT Spawn, whilst the other remains on Site just to make sure they do not fake plant at A.
The point of this setup is that you really don’t need two people watching Short A (usually you might play 1 guy Mid and 1 guy Short). So long as the person watching Catwalk calls quickly, then the CT’s at Long will be able to rotate in time to shut down / delay the rush. Having two players in B and one in Mid for the entire round is a bit of a waste, so the second player at B needs to make sure he rotates quickly. If they seem to be hitting A with everything they have, then you can leave B totally open, and have on guy playing Mid (whilst keeping an eye on B), whilst the other 4 move to defend bombsite A.