scanner coupler/duplexer wanted

I have a Uniden BCT8 mobile mounted radio with NMO mount antenna that covers the lower bands 136-174 and 400-470. I am looking for a coupler or duplexer to add another nmo antenna to cover the 700-800 mhgz bands from the same radio. Cant seem to find a NMO low mount antenna that will do all bands.