As Des 56 says it can be a useful if your "downstairs" lighting protection device should trip you are not entirely left in the dark and vise versa. As been mentioned I'd just put a note next to the consumer board.
This was how we where shown to do it many years ago back in college. Landing light on downstairs lighting circuit, hall light on upstairs lighting circuit. Even more helpful nowadays with RCBOs tripping when a lamp blows.
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