Window splitting was attempted as a 3rd party fork for a short while a long time ago then abandoned (possibly high maintenance to keep split windows under dual control, e.g. when scrolling mouse which has focus etc.)
An old trick was to have split browser frames but that’s no longer possible.
Currently, you can have same or different file in two separate windows open without Tabs
or run same or different file in two copies with/without tabs and several means to control that have been suggested in this forum and GitHub
The latest Pre-release makes this much easier by allowing CTRL SHIFT N to open same file in a New window or CTRL N to open a second New window and you can select any file or open and jump to stored page in Favorites