I have exact same problem, except the radios are located about 3' from each other, which, technically, should not be an issue since wavelength for 900MHz is approx. 12".
The tech support, prior to purchasing the bridges, explicitly said that they will NOT interfere with each other. I don't think it's interference issue, but an issue with one non-root connecting to the root (Master) of the other bridge. So both of the non-root bridges flip-flop between the 2 roots, which causes the range and high packet loss issue.
I do NOT suggest this product for P2P installations, where there are multiple sets of bridges. For single installations, this product is excellent.