One needs to press two strings together with the same finger. I always got problem on this. Don't know if that's because my hand is kind of smaller and my fingers thinner than normal people. I always fail to get good sounds even when I use index finger, not to mention the other fingers. The difficulty becomes even larger at high positions when the separation of the strings increases.
What position do you people use to play the marked parts? For example, the first one, do you play in third position and use middle finger for both E flat and B flat, or fourth position using index finger instead?