Mike Schroder said:
"WP_Image_Editor is built to handle one file at a time, and there isn’t anything built in to combine two images by default.
The main reason here is that the files in-memory aren’t compatible with one-another if different editors (say, GD and Imagick) are used for each.
There are a few different ways to handle it, of course, but one way to solve the problem here would be to load one of the two images, then extend the Imagick editor (or both editors, if you prefer more compatibilty) to support combining images. Say, with a $image->combine( ‘filename’ ) method.
First, you’d load one of the images with wp_get_image_editor() (and specify that ‘combine’ is a required method), then call your combine method while specifying the second image."