Assuming you have your monitors side by side, head into Paint and resize a blank image to 3640x1920. Now paste in your 1080x1920 image on one side, and paste in your chosen 2560x1440 image on the other side (if the pictures you want aren't the right sizes, might need to resize them first). Save this and set it as your wallpaper in Windows.
I have my monitors set up as 3 across with 1 above (all are 1920x1080 landscape). I want the bottom three to span a single image across all of them, and the top one to have a different image. I used the above to create a folder with a dozen or so 5760x2160 images and my Windows simply Slideshows through them. Here's what I mean:
I used to use Displayfusion, which is excellent, but I fancied less software running when I switched over to Windows 10 too.