Presently, if you have two windows open in vertical split mode, obsidian doesn’t allow you to move one of the windows in a horizontal split mode. Either you will have to manually switch to horizontal mode using three dots(which will make a copy window of the first one) and then you open the backlink in the horizontal split.
With the file explorer and the backlink tabs open, not enough room is left to work with the active windows in the vertical split mode.