You should just read up on the laws of AM Mobile Radio Transmitting. It’s a lot easier than experimenting.
Your truck becomes the “ground plane”. The precise placement of the antenna, determines the strength and direction of the signal. For Omni directional transmission, the antenna would have to be dead center in the mass of the truck.
The antenna on the left rear, will project the strongest signal across the right front of the truck.
Over the road truckers wanted the strongest signal, ahead and behind their trucks to chat with other truckers. The mirrors offered a place for the needed 102” distance between the dual antennas, also being centered (front to back) for all practical purposes on the tractor.
A CB with one antenna, will be more efficient than one with two antenna’s that are too close together.
The total answers are just a quick search away on Google.