Not to fly in the face of Warren or John who clearly have more education and experience in radio than me, I'll tell you what I did back in the 70s in Oklahoma. The Highway Patrol was on VHF-Lo while everything else I wanted to monitor was on VHF-Hi and I was not pleased with the performance of the single A/S antenna (MON-31, IIRC) feeding the scanner in the car. I went to a commercial radio shop, bought one low-band and one high-band Antenna Specialists antenna, two mounts w/coax, and a T-connector. I ran both antennas to the T-connector and one short jumper to the scanner and got significant improvement on receiving, especially for low band. I never noticed any reduction in performance or interaction between the two antennas with the scanner. Being in the 70s, the scanner was a crystal-controlled model, a Regency ACT-R-10H/L/U that I still have today.