If possible, separate the the two antennas by at least four feet. The can share the same mast but the closer they are to one another, the more they 'detune' one another.
The two antennas are entirely different in terms of gain, directivity and bandwidth.
The GE Futura is discussed and dissected in this thread at digitalhome.ca:
http://www.digitalhome.ca/forum/showthread.php?t=117594
What are your goals? Are each of the antennas receiving the stations you need/want?