Is it possible to use Safari on OSX with "panes" like vim splits
I am not aware of anything, Safari extension or otherwise that would allow you to split a tab in that way (one over the other, both visible, if I understand correctly). Though I am not the end-all oracle of Safari extensions. Searching Safari extensions repositories (apple or others) would be how I would approach this.

Perhaps there is such an extension for Firefox, Chrome or even Opera that I am not aware of?