There is no way to reverse the rendering order. You will have to either keyframe the top layer manually and rewrite the expression on the second layer to get the animation you want or take a different approach.
I have about a hundred expressions that move layers into position based on the in-point and out-point of a layer. They are not tied to layers above and below. I think they basically do what you are trying to do but all I have to do is to set an in and out-point for all layers and then sequence the layers. This expression takes the position of the layer before the expression is applied, then moves the layer in from the left, it bounces to a stop, then just before the layer ends it drops off the bottom of the frame. It automatically corrects for position and scale. Maybe it will give you some ideas:
sif = effect("slideIn Frames")("Slider");
sof= effect("slideOut Frames")("Slider");
// find in and out point
inTime = sif * thisComp.frameDuration;
if (effect("Match SI SO")("Checkbox") == 0) {
outTime = sof * thisComp.frameDuration;
}
else if (effect("Match SI SO")("Checkbox") == 1) {
outTime = inTime;
}
// Fix layer size when scaled
sf = scale - [100, 100];
xSize = width + (width * sf[0]/100);
ySize = height + (height * sf[1]/100);
realSize = [xSize, ySize];
// Set Positions
spx = 0 - realSize[0] + realSize[0]/2 ;
rstx = value [0];
rsty = value[1];
epy = thisComp.height + realSize[1] - realSize[1]/2;
// create movement
freq = effect("bounce Frequency")("Slider");
amplitude = effect("bounce Amplitude")("Slider");
decay = effect("bounce Decay")("Slider");
posCos = Math.abs(Math.cos(freq*time*2*Math.PI));
nt = time-inPoint;
y = amplitude*posCos/Math.exp(decay*nt);
moveIn = Math.min(linear(time,inPoint,inPoint + inTime,spx ,rstx-y));
moveOut = Math.min(easeIn(time,outPoint - outTime,outPoint - thisComp.frameDuration,rsty ,epy));
[moveIn, moveOut]
I hope this helps you figure out your production problem. I've saved it as an animation preset by selecting the Effects to grab all the sliders and preserve their names and the Position Property. It will work with any 2D layer. There's a modified copy for text layers, and I have another one that is about 80 more lines that work with 3D layers. My idea was to create presets that eliminated the need for any keyframes.