Thanks to all for your replies, very much appreciated!! I suspect what I am trying to do is impossible, but here it is:
1) Pair Helmet 1 to K1600 for music
2) Pair Helmet 1 to iPhone (calls only, no music)
3) Pair Helmet 1 to NAV 5 (directions only, no music)
4) Pair Helmet 1 to Helmet 2 (Intercom)
A Sena rep told me this is possible with the 20 EVO, but from everything I've read, I think the iPhone HAS to be connected to the helmet via the NAV 5. The problem with this set up for me is that the only way I can see to answer a call is to touch the screen of the NAV V. I want to accept the call through the helmet communicator, but to do that, I think I have to give up either the K1600 audio in the headset OR give up hearing directions from the GPS.