On the subject of keying multiple sets of tones with the same data, in the long gone days of the Voice of America RTTY schedules, thoe AFSK circuits on HF did indeed use two sets of tones keyed with the same data.
I can't tell you what equipment was used at VOA locations other than Greenville, but at Greenville, we used Northern Radio Type 153 tone keyers and the Type 174 tone convertors. There were two sets of tones, a low pair and a high pair, 85 Hz shift each. I wish that I had kept notes on the two pairs used, but I don't know the tones frequencies involved.
Another form of RTTY diversity was time diversity. There may be other systems involved, but Barry Research marketed their 6028 system for very critical TTY communications over HF links. I found this link for the BR system:
Enjoy.
73
Sheldon