Answer: What you're describing sounds like what on the violin would be called a fingered tremolo; shorthand for a tremolo on those two pitches, lasting for the time of two half notes. Below I've illustrated that as if it's a measured tremolo in 32nds, but the fact that the second beam doesn't contact the stems suggests it's meant to be an unmeasured flutter. If you're lucky the composer will have chosen two notes that can be played by changing just one finger.