The ground flat stones I linked come in pairs so they can be used together to keep each flat.
I think these ground flat stones are used very lightly on precision flat ground surfaces to begin with, so I would think they will stay
flat for a long time if not abused. A ground flat stone will only hit the high spots very lightly. They're not made to stone off
much material at all. I don't think they're made for a very worn, rough mill table, e.g. Stefan has a video describing them. They're $500/pair!