I have the F9 and I can say that it does not work, we have decided on 2 robots (1 per floor).

My F9 does not support multi-floor. In my opinion, these multi-floor stories do not make sense for devices without a carrying handle (charging station & robot). Besides, 3 units of the F9 cost just as much as a "good" one with similar equipment from a "brand company". It's worth it to equip each floor with a device = longer durability of the devices, no lugging around, all-time operation without walking up and down stairs. I opted for the F9 because, thanks to its lower height, it fits under all my furniture without the laser attachment of the D9.