I've recently moved into a 4th floor flat where the washing machine and dishwasher waste both go into the trap (double nozzle trap) under the kitchen sink. However despite not using both appliances at the same time when either machine empties its waste it is coming back up into the sink, and when I use the dishwasher the waste becomes blocked so the sink will not empty and I have to use a plunger to remove blockage. Waste pipe fitting goes into single stack so am unable to have separate runs of pipe.

Is there something I can do to remove these problems, such as a non return valve for waste fittings?

The piping is either clogged or undersized.