You can change the way Readlang works to stop it from automatically merging adjacent words: Stop adjacent words merging to form phrases. Is that what you’re after?
(If you want a way to split phrases that are already in your word list into two separate ones, then there is no convenient way to do that right now, you’d just have to edit the word and then create a new one. A duplicate option might make this easier but I’m not sure how common a problem this is.)