separated scale dimensions to slider

I'm not sure I completely understand what you are trying to do. 

If you want to ease from one keyframe to another or ease over time you need to define the time of the keyframe and the duration of the easing. 

For example:

t = time - key(time);
v1 = ease(t, 0, 1, 100, 200);
v2 = ease(t, 1, 2, 200, 100);

value + V1 + V2

This expression will start changing values at the start time of the first keyframe and change the value from 100 to 200 over 1 second; then it will ease again from 200 back to 100 over the next second. 

It looks like you want to use time as t, start changing the first variable when time passes the first keyframe, and keep changing the value until time equals the second keyframe. I don't see any reason for the if statements. I think that something as simple as this would do it:

t1 = key(1).time;
t2 = key(2).time;
t3 = key(3).time;
t4 = key(4).time;
t = time;
def = value;
nVx = thisComp.layer("Null").effect("A")("Slider") - def[0];
nVy = thisComp.layer("Null").effect("B")("Slider") - def[1];
x = easeOut(t, t1, t2, 0, nVx);
x2 = easeIn(t, t3, t4, nVx, 0);
y = easeOut(t, t1, t2, 0, nVy);
y2 = easeIn(t, t3, t4, nVy, 0);
if (t < t3)
	 def + [x, y]
else
	def + [x2, y2]

If you apply that expression to the scale of a layer, and the layer starts out at 100, 100 and the Slider A is 150 and slider B is 160, the scale will ease from 100 to 150 between the first and second keyframes, then hold until the third keyframe and return to 100 in x. The Y value will start at 100 and increase to 160 by the second keyframe, then hold until the third keyframe and ease back to 100 in y. 

Is that what you want to accomplish?

So that is the expression I've been able to get to work:

if(numKeys>1){

t1 = key(1).time;
t2 = key(2).time;
t3 = key(3).time;
t4 = key(4).time;
v1 = 100;
v2 = thisComp.layer("Null").effect("A")("Slider");
v3 = thisComp.layer("Null").effect("A")("Slider");
v4 = 100;

if (time < t2){
easeOut(time, t1, t2, v1, v2);
} else {
if(time < t3){
ease(time,t2, t3, v2, v3);
}else{
easeIn(time, t3, t4, v3, v4);
}
}
}else value;

The question is it possible to add some easing to the animation? As it seems that the expression ignores the original easing and the animation is completely linear.