I think the addition of Split Views is a great improvement for productivity, but I think there is definite room for improvement down the line. Things like horizontal splitting may be helpful, but I think a more useful option would be pinning one tab to one side and allowing users to switch the tab split to the side.
In terms of keyboard shortcuts, I think they could just assume if you're tabbing between them in the current order (e.g. tabbing in order will swap between the tabs either side of the two tabs but still allows interacting with the main pinned tab too), though I think a shortcut to focus on each tab view would be useful as way to be able to switch to a tab that isn't immediately next to it, but still interact with each side without having to sift through all in-between the pinned tab and the other tab.