Don’t mix up the transmitting frequency (“out”) with the receiving frequencies (“in”). When you state that a lot of U.S. aircraft got dual-frequency boxes that 978Mhz +1090Mhz combo only applies to the “in” part. The frequency saturation issue, if any, would only be applicable to the single “out” frequency chosen, not the “in”. If the “in” frequencies mattered it would be like saying the more television antennas on the roof tops of apartments in a city the more frequency saturation. ADS-B ground stations and aircraft rebroadcasting of data could care less how many airplanes are receiving that signal (just like a TV station’s transmitter broadcasts in the blind). And as for “out” data transmitted by the approved onboard aircraft ADS-B equipment the FAA does not allow both frequencies to be used for transmitting from a single aircraft. FAA specifically states not to have two “out” transmitters onboard. It’s a pick one or the other “out” frequency and stick to it. So an aircraft either transmits on 978Mhz or 1090Mhz but should never legally have both.