Sink downstairs overflows from upstairs sink?

A blocked pipe. The water can not go down the pipe, so it comes out in your sink.

All upstairs plumbing fixtures drain through vertical pipes in the walls, connect to the horizontal pipes under the house and continue downslope to the city sewer or your septic tank. A backup into a...

A clogged drain most likely.

yes, by eating it

Most of the one's I have run across do not have an overflow. Mainly used for disposal, dumping out drinks, etc, not for washing so overflow not really an issue. Also, most are stainless which do not...