The VHF performance of the 91XG is quite poor relative to it's excellent UHF capability. In weak signal conditions such as yours, the 91XG can't be expected to offer reliable reception of any real channel ranging from CH-2 to CH-13.
The CA8100 on the other-hand, is quite capable in the real channel 2 through 13 range. Have you considered pointing the CA8100 at 142° magnetic? Reception of real CH-11 should be fairly easy for the CA8100. Presuming that's successful, point the XG91 at 346° magnetic to receive the UHF (real CH-14 and up) signals. Combine the two antennas using a UHF/VHF combiner. The UHF capability of the CA8100 would be blocked by the filters at the VHF input port on the combiner and any VHF signal sneaking in via the 91XG would be stopped by the filters on the UHF port of the combiner.
The antennas Direct EU385CF is one example that also includes a weather resistant housing.
http://www.amazon.com/Antennas-Direc.../dp/B008PBTPN4
Solid Signal sells a functionally similar product with no housing.
http://www.solidsignal.com/pview.asp...zhyBoC72Lw_wcB
Radio Shack might still have some UHF/VHF combiners in stock... If their doors and windows aren't boarded up yet.