Script that takes 2 images and overlays one on the other

I have a folder with 200 raw images in it. 100 of them might be called 1001-1001-1072 (Yellow).jpg and it's sister image would be 1001-1001-1072 (Yellow)_Overlay.jpg - making 200 images. The next image in the series might be 1001-1001-1073 (Red).jpg. It's really the last string of numbers that the script should look at.

Basically each image will have a sister image that will be used in photoshop as an overlay layer (of a script defined percentage). Can one of you geniuses create a script for me that will do this to all the images in a folder?

I'd like to use this inside of an action, if that means anything in the script parameters...

Thanks in advance for trying -

Ian