abnormal short burst cistern flush of a continuous cycle?

Should not be possible for one to syphon the other, The soil stack should continue up over, to vent the stack, open top,(covered to stop water getting in, but open) Open, means syphon not possible, But something to check, especially if the property has been extended.
Start with downstairs unit, Has the valve seal gone, and the water not turning off fully, once float is high, water should stop going in, if not new seal required tie float unit up high, to stop water going in. Do not use, leave off, Then see what happens with upstair unit, Same check on upstairs unit, does water stop, or keep running with float high.

SD
January 2012