Many years ago, antenna manufacturers, such as Channel Master, offered stacking bars for several of their antenna models. I still have a brochure from Channel Master showing the difference in db gain of their UHF Super Bandsaw, Model 4310, comparing single to stacked configurations.
Since UHF antennas would be stacked close to each other, and high-band VHF antenna stacking would be practical, I don't know why manufacturers aren't offering stacking bars anymore. Stacking bars eliminate spacing error and combiner losses while making it simple to stack two identical antennas.
I have contacted AntennaCraft requesting they start offering stacking bars for their YA10-7-13. The more requests they get, the better chance we have to get such a product. Should be simple and fairly cheap to manufacture.
Is there a market for stacking bars?