It shouldn't be a problem to have two entities controlled by different players.

Here is an example code that should work (e.g. for a splitscreen game):
Code:

---

entity* player1;
entity* player2;

action player1_act
{
  player1 = my;
  //here comes the rest of your movement code for player 1
}

action player2_act
{
  player2 = my;
  //here comes the rest of your movement code for player2
}

---

Now you should be able to get the x-distance between both of them and store it into a var:
Code:

---

var distance;

//put this into a function:
distance = abs(player1.x - player2.x);

---

But you have to make sure that both entities exists and that their pointers are not zero.