Combining two antennas is easy… and you might consider doing it if you have channels in two different directions or want to use a separate VHF and UHF antenna. Combining antennas gives you ultimate flexibility, and if you’re a homeowner, there’s no one to stop you.
It’s actually pretty easy. If you’re combining two of the same kind of antenna, just make sure they’re pointed in totally different directions. Pretty much any antenna splitter can be used “upside down” as a combiner. If you are combining VHF and UHF antennas, choose something like this Pico Macom combiner that isolates the different signals.
No matter what you do, there’s one rule that is absolutely key: Make sure the cables from each antenna are exactly the same length. I mean, down to the millimeter if you can do it. If you use cables that are two different lengths and there are any signals that come from both antennas, you’ll have “phase problems” meaning that the combined signal will be a ton worse than the original antenna signal. Essentially it’s like creating audio feedback by pointing a microphone at a speaker (it’s actually a little different but not the point.)
It’s easy enough to measure out the cable lengths and make sure they’re the same, and believe me, you’ll be glad you did.