I own a two family flat. The upper flat is occupied but the lower flat is vacant. Whenever the upper flat uses the kitchen sink it drains down to the lower flats sink and then onto the basement floor. For instance, when the upper flat lets the water out of the sink you can see the suds in the lower flat kitchen sink. Then the water is coming up in the basement of the lower flat. The basement of the upper flat is dry.

Whenever the upper flat uses the kitchen sink it drains down to the lower flats sink and then onto the basement floor. Sounds like a blockage downstream from the basement floor drain, The solution would be to snake the basement drain line.

The basement of the upper flat is dry Are the units stacked or side by side? Good luck, Tom