My general recommendation with relations: If you can do something without a relation, do it that way. A lot of people don't understand them and some editor software can't manage relations well, so there is a big chance that someone will mess it up accidentally in the future.
Maybe you can tag on the restaurant that outdoor seating is mapped as a separate element with outdoor_seating=separate
. It's not used frequently, but separate
means that something is mapped as a separate element.
If you still want to do a relation you can use a site relation. You can use that for any kind of POIs. I don't recommend this though for 2 members. Example. Overpass query for all site=restaurant
: https://overpass-turbo.eu/s/1QjJ
If you afraid someone will forget to change the operator
, the same editor can forget to update the relation as well, so I think you can't win this battle.