So I see the icon for split horizontally at the top where it says which View we’re currently in. Can we add other icons there? Or change it? I would prefer Split Vertically to be there, and I can’t figure out if I can add it.
No there isn’t a way to change or add the icons here.
You can change it quickly to the Split Vertically icon temporarily by holding down the Alt (Windows) or Option (Mac) key when your mouse is hovered over the icon. This allows a quick way for you to choose to split vertically instead of horizontally.
You can also split vertically via the View->Split View menu which also lists the keyboard shortcuts for this functionality.