As stated above, when mixing two materials with differing amounts of rounded edges, the shader just chooses the value from the first input.
As can be seen in the attached images the blue metal has edge roundness, even though the materials roundness is set to 0?
The reason i am trying to blend two different round edges values, is to try and create variation (using noise for example) since rough metals usually don't have completely even edge roundness all over. If there is a better way to achieve this please do let me know .