If you had followed the steps in my previous post, by now, your nemo will be able to select a file and compare it with another file. If you think it should also have the ability to compare multiple files when you select them, Go ahead. Read on!!!
Now, you need copy the content below to $HOME/.local/share/nemo/actions/compare-multi.nemo_action
Name=Compare selected items
Comment=Compare %F saved file or directory.