Hi guys,
Speaking from 40 years of experience, one of two things, likely a combination of both is happening here. First, the scanner is being overloaded by the CB transmit and second a CB usually puts out a lot of crud even if unmodified and no big "leenyar" is contributing to the problem. I have seen CBs radiate harmonics from the case which of course can't be filtered plus all sorts of spurious emissions that readily show up on a spectrum analyzer. While they must meet FCC certification standards they seldom actually do, I found this out when working with the FCC on interference complaints. In any case, listening to both at the same time would drive me crazy, transmitting is worse, one is a distraction to the other.
A common misconception is "co-phasing" two mobile CB antennas. Antenna Specialists was the only company to actually accomplish this with a system that electronically rotated a signal using two identical antennas spaced 9' apart on the vehicle. They had to be mounted "fore and aft" to work properly, any other spacing totally upsets the apple cart. BTW, it worked at it's best while "mountain topping", that is stationary. While moving it's too hard to stay pointed in the right direction which is why an omni position on the phaser was provided.
I laugh myself silly when they fall for the "Twin Trucker" snake oil hype that proports to make an omnidirectional pattern out of a figure 8 created by the vehicle body. Without getting all techie about it, let it be sufficient to say it just can't be done with any sort of predictable results. My OM fell for this nonsense and put them one on each side of the trunk of his '63 Olds Holliday. When I inherited it one came right off and nothing had changed signal wise, the pattern still fired off the front of the car. BTW, they're a booger to tune like that, not worth the effort when absolutely nothing is gained.
