If adding reception of WPVI-ABC, real channel 6, and WHYY-PBS, real channel 12 is the goal, increasing the height of the antenna is a logical next step.
The ANT3036WR is generally the correct type antenna, it's designed to receive real channels 2 through 69.
Reception of real channel 6 is more difficult due to higher noise levels at that frequency.
If this fails, you're facing a decision to consider a more capable antenna such as the Winegard HD8200U and possibly higher mounting options such as a tower.
Combining the signals from the ANT3036WR and another antenna will almost always produce less reliable reception. Use of a second antenna is usually done with the aid of an A/B switch.
Another possible solution for using multiple antennas involves an additional set-top-box tuner. SD tuners are fairly cheap. HD tuners cost a bit more...
Channel Master and
Pico Macom are a couple of examples of HD capable STBs. With one antenna feeding the existing TV tuner and the second antenna feeding the STB, the TV remote will usually let you select the HDMI or video input of the TV.
In your case, you would potentially need a DVR dedicated to each antenna.