The HD-8800 UHF antenna comes with the mounting hardware that will allow you to attach the antenna to your 1.25 mast. You can mount the HD-8800 antenna 12” below MS-2002 antenna. Hopefully the HD-8800 will be high enough to obtain a good strong signal on channel 21.1. The HD-8800 antenna is not an amplified antenna by it self but when you install the AP-4700 antenna preamplifier to amplify your signals from Harrisburg you will have a power injector and wall transformer just like that used with the MS-2002 antenna. You are using 2 separate antennas and you are NOT stacking antennas for signal gain (increase) so the lead lengths do not need to be equal lengths.
The CC-7870 is a passive antenna coupler and does require power nor does it care which direction it is pointed since it is not an antenna and does not receive a signal out of the air. The CC-7870 coupler will couple the output of the power injector from the MS-2002 antenna and the AP-4700 preamplifier to form a single combined output lead. The combined output signal will then feed your existing splitter system.