That gets complicated. When you split a coax to two identical antennas a number of things can happen. If they are at the perfect spacing and pick up the same signal in phase, meaning both antennas see the same wave front at the same time, then you can get up to 3dB gain over a single antenna.
If the two antennas do not see the same signal equally, as in around a tower, then you can have anywhere from a little gain to lots of loss. You can start with about 3dB loss in the splitter then if the antenna spacing causes phase cancelation the loss can go through the roof. For example if you have two dipoles at 155MHz spaced 3ft apart and in line with the transmitting station where the signal will hit one antenna first then the next antenna 1/2 wavelength away or 180 deg out of phase, it can cancel 20dB or more. The signal hitting both antennas can be very strong but most of it can disappear due to phase cancelation. Add a third or fourth antenna it gets even more complicated.
So if you want to experiment with multiple antennas around a tower, get ready for a little fun and a lot of disappointment. If you want a hassle free install just put the antenna at the top of the tower and in the clear. Or if you only need reception in one major direction using the tower as a reflector can be your friend.
