There is no obvious way to open note in horizontal split. All the available Ctrl/Cmd-click and Enter hotkeys are opening the file in vertical split.
The only way I have found so far is just to perform horizontal split and then open a note (not ideal, because the currently active note moves to the lower pane).
For now yeah, just split pane and shortcut to open in horizontal split. We’re expecting most people to use vertical split (especially for opening linked panes).
We do have plans for the next iteration of pane system to allow arbitrary drag-n-drop reordering of panes, and with that you’ll be able to easily rearrange it.