This is also true of having more than one antenna on the same pole, even if they are pointing in different directions they are still going to interact with each other, usually in a negative way.
As far as using a splitter as a combiner as already stated, there is no free lunch. There will be a net negative effect regardless of how careful you are. For example, when you connect one antenna directly to your digital box (or DTV) and say you get a decent signal quality, you will almost certainly realize a loss when you connect anything else up. This loss may or may not be enough to effect your television viewing. A lot of it depends on location.......location of you relative to the TV transmitter, the location of your antennas in relation to each other and many other factors such as signals bouncing off of or being blocked by buildings etc. as well as many other things.
With antenna systems, especially with all the changes we're seeing at the present time there will be a lot of trial and error required to figure out what is sufficient. For me, I'm thinking the only logical way to go is to have an outdoor antenna on a rotor. The main reason being is the distance from me to the transmitters and the fact that they are in many different directions. It used to be in this area there were a handful of known OTA channels and the method to receive them was pretty much the same, one UHF antenna for the major networks which all transmitted from the same area so one ant. did them all and one VHF antenna for one other local broadcaster. The only variable was your distance from the transmitter. The farther you were from the tower, the better the antenna. If you had the best antenna you could find then you added an amplifier. Failing that you were S.O.L. and satellite was the only fix.
Now things are different and some trial and error is definitely in order.
