Combining antennas is always a trial and error situation. There just is not a reliable and repeatable method to do so.
The signals do all kinds of strange things, they get reflected and bounced off buildings, the atmosphere, the ground, roofs, split apart by tree limbs and other objects then they re-combine behind the object, and so on.
Just the placement of one antenna can, (and often is), trial and error. To try to combine them is more so.
Here is some good reading on the subject. (At least it helped me understand the issue better.)
http://www.hdtvprimer.com/antennas/merging.html