Multipath is when the same signal is recieved factions of a second apart. can be from a reflection off something, even a mountain (think echo). what I think was happening was you were getting the closer channels recieved twice, a fraction of a second off, then when you combined the antennas, it made a signal the tuner couldn't lock onto. The yagi is probably the only antenna you have that'll get the signal from the far stations, and the other should be fine for 14 miles. I doubt the signals would combine enough to cause a tuner to malfunction.
in analog, multipathing results in ghosting.
Bryan Socha
Media Center MVP 