I have an ABC affiliate north west of me in Santa Barbara on real channel 27 while the Los Angeles stations are 65 miles to the east, about 160 degrees separation between the two. I had a hard time picking up the Santa Barbara stations with my AD Clearstream 4 antenna pointed towards the LA stations. Adding another antenna pointing toward Santa Barbara degraded the signal on several stations from Los Angeles.
I found a site that told how to build a directional yagi antenna tuned for a specific UHF channel (pdf file:
http://www.mtmscientific.com/yagi.pdf). I built one tuned to channel 27 and, I guess because it doesn't pull in other signals very well, was able to combine it without degrading the Los Angeles stations.