Automation in Photoshop

The automated process would need to use some logic to be able the pair up the two Landscape or two Portrait images to populated into one or the other template.  You need to design some pairing operation process to order the population of your templates

Replies to your points, with questions where needed:

1) Open Photoshop 

1) With the right code additions, launching the script from the Mac Finder or Windows Explorer can open Photoshop and make it the active/front app.

2) Select Location for Images ( All Images of same Size 4"X6" or Size 6"X4")

2) This is scriptable, will the source folder only contain images, or sub-folders? Are all images in the same file format, or will there be a mix of say JPG, PNG etc?

3) Place Images in Template ( Page with Letter head, and space for 2 images)

  Note Template is Selected Either Template 1 or Template 2 based on image size 4X6 or 6X4

3) Do you want the template auto selected by the portrait or landscape orientation of the files in the input folder in step 2? Will the input folder always have the same orientation images in it? What happens if there are a mix of portrait and landscape orientation images? 

Or do you mean that you manually select the appropriate template because you know the orientation of the input folder contents?

If there are two images per letterhead template, will the input folder always contain multiples of 2 images?

4) Save the Result Template with JPEG or Print as PDF and save

4) Do you mean save as Photoshop PDF format using JPEG compression? What quality level?

Do you  know how to program in JavaScript?

If not, then I would suggest that you make use of a pre-built script and workflow, such as:

http://www.mouseprints.net/old/dpr/PhotoCollageToolkit.html

BatchMultiImageCollage.jsx