One of the guys I talked to, suggested that using 2 into 1 would alter the resistance of the cable into the head unit and that would reduce the reception. No idea if this made sense...anybody agree with that?
There is no doubt that running both antennas will change the resistance and affect the performance. Any additional cable, antenna added increases the load that the head unit will see. Think about the loading of a 2ohm speaker vs. a 4 or 8ohm speaker. Huge difference depending on what the receiver is optimized to work with.
What I don't know is if this will increase or decrease performance on the bike. And this could vary greatly depending on the strength of the signal that the antenna is receiving. Having more antenna doesn't necessarily increase performance.
Wish I was back in my engineering days when I could access a test range to try that out. I sure there are some engineer types in the group that may be able to give specifics.
I would also love to see the pics of how the hidden antenna was mounted to maximize performance. I was considering adding this when I when I installed my gps unit. All I really care about is the WB performance - Ipod used all the way.