You absolutely do not need to dump your XG-43: it is an excellent antenna, and a MUCH better antenna for UHF than the HD-1080 is. I suggest that you ADD a Winegard YA-6713 or an AntennaCraft Y-5-7-13 aimed in the same direction as the XG-43. Be sure to separate the two antennas by at least 4' on the same mast, and I suggest that you mount the XG-43 ABOVE the VHF yagi. Combine the signals from the two antennas using a Pico-Macom UVSJ antenna joiner. Add a small distribution amplifier like a Channel Master CM3410 in the line before your splitter, and you should clear up any problems that you are experiencing.
http://www.antennacraft.net/pdfs/Y5-7-13.pdf
http://www.winegard.com/kbase/upload/ya-6713.pdf
http://www.solidsignal.com/pview.asp...Combiners&sku=
http://www.solidsignal.com/pview.asp...mplifiers&sku=