Your opening post suggests you have combined the two antenna using a reversed splitter. That often produces mutual interference. (A weak signal from one antenna interferes with the better signal from the other antenna.) You may be trying something we all wish would work... and have proven time and again, doesn't.
Have you tested each antenna for reception quality, alone, (the other antenna and splitter removed from the setup)?
I agree, the big trees can't be helping.
If you want to use two antennas, consider:
http://forum.tvfool.com/showthread.php?t=2882
The bottom line is, you need to adjust one antenna at a time, with the other antenna completely disconnected. A reversed splitter is rarely successful as a multi-antenna combiner.