How do i split a viewport horizontally/vertically?

Hi Lilly-

Unfortunately, the SplitViewportHorizontal and SplitViewportVertical are not yet supported on Rhino for Mac (you can find a complete list of currently unsupported command here).

To deliver a bit more bad news, when you drag the vertical divider between the viewports, it resizes the viewports on both the top half and bottom half of the Rhino modeling areas. (You will notice this is not the same as the Windows behavior and also not the same as dragging the horizontal dividers in Rhino 5 for Mac). This is a known issue and it limits the sizes and shapes of the viewports to some degree. We don’t have a fix for this coming in Rhino 5 for Mac; we are investigating a fix in Rhino 6 for Mac.

NewFloatingViewport might be a good work-around in your case.

I hope that helps,
-Dan