[time-nuts] Advice on sighting a roof mounted gps area please

[swingbyte]

Hi All,
I am building a house extension and part of the works involves adding a 
new hip roof made of corrugated iron.  I was thinking I would pass a 
50mm pvc pipe through the roof with a tee and then mount two conical gps 
timing antennas on top of it.  I am in a low point and don't have 
visibility of the horizons ( I'm not in the out-back).
My question is should I mount on the peak of the roof?  How close can I 
mount two antennas from each other? Can they interfere with each other?  
I am also in the midst of some tall trees - although my new roof will be 
pretty high it will still be below the tallest trees.
Of course the main reason for this is I want to do some accurate timing

ASCII art of proposed set-up

   A                  A
   |                  |
   --------------------
            |
            |
            ^
           /  \
          /    \
         / roof \

Thanks for your advice

Tim