I plan on installing a NMO CB antenna and a NMO 800 mhz antenna on top of my truck. The CB will be TX/RX of course, but the 800 will be RX only, does it matter how I place the antennas on the roof?
Try to place them in the center of the roof and as far away from each other as possible to avoid possible receiver overload when you transmit on the CB. You'll know it if it occurs as you will cut out your 800MHz radio reception when you transmit on the CB. You may have no problems at all since the spread of frequencies is far apart and if you keep the CB to the legal power limit.