splitting an image with aspect ratio

It's not cutting it in half, but is this what you need:

Set the rectangular selection tool to 8 X 10 ratio.

Make a selection of one part of the image, crop, and save it.

Undo back to the original image.

Repeat for the other image.

I do this with an action for my digital scrapbooking pages. I create pages that are 12" x 24" and then crop one side to 12x12, undo and crop the other side to 12x12. Each time I save the cropped version as a copy. Your image must be set up like didiermazier said - 16/10 ratio. 

Using your crop tool, set it to 8x10, place it on the left side. Crop. Save a copy. Undo, repeat on right side. If you have a bunch, turn it into an action. 

hard to answer not seeing your image…

But mathematically if youwanna split int 2 with each a 8/10 ratio, your image must first have a 16/10 ratio.

Then you can use guides setting with 2 columns.

Then select each half  copy and new doc then paste

i.e left part new doc: