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To minimize stress on the rotor and maximize its life, it's always best to try and minimize the length of the mast above the rotor. The stress levels go up approximately with the square of the length of mast, so any height you can eliminate will really help.
Just a few comments:
1) If all you want is the 91XG and the YA-1713, then you can probably get away with less than 10 feet of mast. If you keep about 4-5 feet of separation between the antennas, you can probably get away with about 6 feet of mast.
2) The 91XG and YA-1713 are both relatively light-weight antennas. As long as the mast above the rotor is kept short, the CM 9521A should work fine with this kind of load.
3) For taller masts or heavier antenna loads, you need to use a rotor alignment bearing and guy wire stabilization above the drive unit.
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