Atomic Filament is no longer distributing the model for their spool splitter, for liability-ish reasons, so I made my own. Originally inspired by https://www.flickr.com/photos/68272764@N05/49406556906/in/photostream/, but adjusted a bit.

You'll need to print two of these. Quite a bit of force is required, so no guarantees this will not break your spool.

Source code at https://gist.github.com/apexskier/a676e723c1386723677b633cfa137e61.