is it possible to program a game that allow user to switch player as in the game level, for example the 2nd character follow 1st character as a dummy and user can switch to use the 2nd character?

if yes, how can it be made?

yes it is possible.
You could use a state_skill, a skill which represents the status of the entity.
Example: if its control by player the state_skill is 1, if not its 0

Now in the entities while loop you have to check which state is set and if the player controls it, set the cam to it + move it by a move vector which is set via the control keys. (basic movement)
If it is not controlled by the player, you could two things:
a) just turn towards the player and run
b) use pathfinding to get near the player

Another possiblity would be, if the player changes from character A to B:
-remove player_a, remove computer_b
-create computer_a, create player_b


You could also implement "orders" so the player could decide if the other character should follow or hold his position.

okei....
so i need to define state skill
at begining player1.state_skill=1
and for example if "q" is press player1.state_skill=0 and Player2.state_skill= 1
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entity* computer;
var vec[3]=0,0,0;

action player_action{
my.gravity = 3;
my.z_offset = 6;
player = my;
ent_create("player2.mdl", you.x, computer_player);
ent_create("player1.mdl", my.x,);

if(key_q==1){
if(player1.state_skill=1 && Player2.state_skill= 0)
{
function player_selection{
ent_remove (my);
my = ent_add ("player2.mdl", my.x);
ent_remove(you);
you = ent_add ("player1.mdl",you.x);

}
else if(player1.state_skill=0 && Player2.state_skill= 1)
{
function player_selection{
ent_remove (my);
my = ent_add ("player1.mdl", my.x);
ent_remove(you);
you = ent_add ("player2.mdl",you.x);

}
}
wait(1);
WHILE (1) {
handle_gravity();
handle_movement();
handle_camera();
handle_animation(1);
wait(1);
}
action computer_player{
computer = me
while (computer !=NULL)
{
me.vec[0] = you.vec[0]-60;
me.vec[1] = you.vec[0]-60;
if(key_g ==1){
me.vec[0] = you.vec[0]+60;
me.vec[1] = you.vec[0]+60;
}
if(key_b==1){
me.vec[0] = you.vec[0]-60;
me.vec[1] = you.vec[0]-60;
}


}

------------------------------------------------------------------------------

and i use a invisible box in WED to fix the starting location for the player?
by the way do u know any tutorial that can learn about the order the 2nd character to follow player to attack same anemy, attack nearest enemy or even pick up item?