When I have two tabs in differenet rows (after doing View -> Layout -> Rows: 2) - Is there a shortcut (or a way to create one) to switch between these two?
Like Ctrl + Tab is switching between two tabs in the same column.
The View -> Focus Group -> Next menu item will switch between them. Like all menu items, the relevant keybinding is shown on the right hand side.
If you want to use a different key combination, you can open the ST Console (View -> Show Console) and type sublime.log_commands(True) Enter and execute that menu item again to see what command and arguments it uses. Then, it's just a matter of editing your user keybindings accordingly.