In actions, how do I open 2 separate images then merge them in one image and save them?

I have around hundred's of images, these images are to be paired, eg, Image 1 & Image 2  are  to be opened, merged together and saved as one image., simillarly Image 3 & Image 4 are to be opened, merged together and saved as one image, and so on. 

I have created an action, but when using the action in automated batch, only one image is opened, so my action doesn't work. 

I have loads of images which are to be processed in similar manner, I'm looking for an automated aciton for completing my task. Can anyone help me with this? I'll really appreciate.