Hans Schneeberger writes in his book about SBB locomotives that the diagrams of Ae 4/7 didn't include any regular running South of Gï¿½schenen after 1965.
All international trains of those years that I recall always had a domestic part and if it was only the dining car, an AB Leichtstahl and perhaps a mail van being detached in Chiasso.
I do remember international trains that were too long for any station, even Zï¿½rich HB around 1980. But there were parts of the trackwork filled with sleepers/ties or similar, so that the first cars were accessible. Well, at that time a platform hight of 0 cm above rails was still "normal".
Unlike the Re 4/4 II with its MU possibility, there was no reason to plan two Ae 6/6 for the same train.
And don't forget, the last Ae 6/6 was delivered in 1966, the first series of Re 4/4 II in 1967/68, 1969 the delivery of more Re 4/4 II started, 1971 the Re 4/4 III came and 1972 the first Re 6/6. It was a time of fundamental changes and I don't think 1970 at the Gotthard was the same as 1969!