Back in the late 50's and early 60's, some cars did have dual antennas from the factory. The late 50's/early 60's Desotos sometimes had them. It might have been something someone added as I don't think the A bodies ever had this feature fromt he factory.
The way they're set up they could be a collinear array, but I expect that it's more a style thing. If they were a collinear array then there'd be some kind of comm gear in the car, and my guess is that you'd find screw holes or wiring harnesses to somewhere that normally wouldn't have such.