The 4221 has a pretty wide beamwidth, it is 45 to 50 degrees, I don't know where the 10 degrees came from. Even the CM4228 has about a 15 degree beamwidth. Because of the way the CM4221 and CM4228 are constructed they have better pickup from the rear than a UHF yagi but it's not something I would rely on - worth experimenting though. I've seen posts from people with CM4221 s getting great results from stations at 15 miles exactly 180 degrees away from the direction the antenna is pointing. And the only UHF antennas that have any decent VHF capability are the CM4228 and the antennas direct 91XG. Most of the others are worse than "rabbit ears". The CM4221 can be OK if you have close-in VHF stations but not for any distance.
The CM4221 and CM4228 are not good at rejecting multipath, partially because they do have a wider beamwidth than UHF yagis. The 91XG is excellent at reducing multipath partially because its beamwidth is very narrow (less than 10 degrees).

Coupling two antennas is a lottery at the best of times, unless one is VHF and the other is UHF. The signals from any given station arrive at the antennas at slightly different times and so the signals are out of phase with each other when they get to your receiver. This is essentially the same as multipath - in the worst case you end up with no reception at all. Antennas with decent pickup of the back of the antenna, and with wide beamwidth, or pointing at 180 degrees to each other, are worse when coupled together than narrow beam-width antennas. And of course coupling an omnidirectional antenna to another is the worst scenario.

However, many people have successfully coupled antennas together, it is entirely dependent on your situation and pretty well impossible to predict.

FYI this [Link: hdtvprimer.com] is a useful site when looking at antennas.