A header transition connector includes a header housing having a separating wall separating a first cavity from a second cavity. Header signal contacts are held by the header housing. The header signal contacts are arranged in pairs carrying differential signals. The header signal contacts have first mating ends in the first cavity for mating with a first receptacle connector. The header signal contacts have second mating ends in the second cavity for mating with a second receptacle connector. Header ground shields are held by the header housing. The header ground shields have first mating ends in the first cavity for mating with the first receptacle connector. The header ground shields have second mating ends in the second cavity for mating with the second receptacle connector. At least a group of the header ground shields are electrically commoned with each other within the header housing.