acsutils uses a simple convention for functions returning multiple values, like RotateVector():

There are global variables r1 through r8. The functions write their return values to them.

  RotateVector(x, y, angle); // Call the function
  int newX = r1; // Get the first returned value
  int newY = r2; // And the second one

Warning: you should retrieve the return values AS SOON AS POSSIBLE, or else they may get overwritten by other functions that return multiple values.