Okay, so effectivly you want to slide an alpha channel mask across the images, but the images don't move. They slide blend across.
1st you need a mask image 3 times wider than your image, left is all black (full transparent), center ramps from black to white, right is all white (full opaque). Starting with a 5 pixel wide image (2 black, 1 grey, 2 white) BilinearResize it up to the right size should give the right effect. Use a variant of the Animate code above to make a clip that has the slide effect.
Code:
___ white
/
/
___/ black
| | -> slide
Using the 3 clip form of Overlay should then be a no brainer.
I'll knock up a suitable script when I get home if you like the sound of this.