Hey Jesus,
Personally, I'd have a separate upload field in the section that stores only the WebP images. So there is one upload field that stores the jpeg images and one upload field that stores the webp images. I'd ask CMS users to ensure that the webp images are in the same order as the jpeg images in the upload fields to make displaying them easier.
Then, on the front end of the site, I'd loop through the set of jpeg uploads and display the linked webp images together inside of a picture tag.
The great thing about this method is that if you were to build a plugin that automatically converts the images from jpeg to webp in future, you wouldn't need to update the front end code or the CMS sections. You could make it so that the plugin converts the jpeg image to webp, then saves it in the webp upload field.
Cheers,
Greg
Greg Thomas
PHP Programmer - interactivetools.com