Svenestron,
Yes I believe it is to reduce the interaction between the tip vorticies but it has been a looooong time since I went to school on that machine so I wouldn't swear it to be true. I can say that the sound is much lower frequency and amounts to a low hum, much quieter than the unit on the blackhawk which is about the same size aircraft. Not only are they space apart axially but they have an odd phase angle separation (the blades are not all 90 degrees apart). This may have as much to do with the low noise as anything else.
I'll be interested to see if the B model EC145, whenever it comes, will have the fenestron tail on it. I don't think the machine has much of an issue with tr performance as it is so any change to the fenestron would most likely be for noise reasons only.
Max