I believe you're talking about a preamp which has a amp out right near the antenna which is powered by the power supply into you're house. Connecting two antennas using a coupler then through the preamp won't damage your receiver.
However it can cause reception problems if the antennas are faced in different directions. In all depends on the angles, type of antennas used and channels you're trying to pickup. As a general rule you'll get ghosts and phase problems because both antennas can pick up the same signals even though one is facing a different direction. If the antennas receive the same channel from two different paths (time delayed in microseconds) then you get ghosts. If the two antennas see the same station at the same time but with different gain levels then electrically one signal can substract gain values from the other. The best way to avoid interferrence when combining two antennas in different directions is if the channel frequencies are very far apart. Example north is channel 7 and you have a channel 7 yagi antenna and south are some UHF stations which you have a UHF only antenna for. Sometimes traps or bandpass filters are added to suppress other frequencies. In a nutshell the more different the antennas are the better the chances (example a VHF only and a UHF only facing in different directions).
Let me know what type of antennas you're using, which channels for source A versus source B and are the expected signal strengths close to each other (example are they about the same milage)?
