Tone frequency spacing is large enough with 850 shift to provide significant diversity effect, and many military and commercial FSK modems are designed to take advantage of this, automatically providing copy on one tone in the absence of the other.  When this is combined with another similarly capable modem in space diversity, really excellent fade immunity is obtained. 

Combination time and frequency diversity modems such as the MD-1142 mentioned by Brooke Clarke are even more effective.  These were available surplus for a time, and several of us have them and have experimented with them.  THese 8-channel modems provide remarkably low error-rate performance for non error-correcting baudot teleprinters, even in disturbed conditions.  The simpler two channel frequency diversity mode of the MD-522, which provides two 85-shift channels separated by about 2.4 kHz, is also effective.  The more modern military serial and parallel tone PSK modems all have bit-interleaving, thus providing a time-diversity effect.  

I would be surprised if the current sound card based modems designed for amateur use include in-channel tone diversity capability, as most are oriented towards 170 shift, where little diversity effect occurs.  Perhaps someone on the list familiar with the inner workings of these sound card modems can tell us whether any are capable of in-channel diversity on 850 shift...

73,
John K9WT

On frequency diversity, as shown at
https://w6iwi.org/script/csvgraph.html?CsvUrl=https%3A%2F%2Fw6iwi.org%2Frtty%2FTuNotes%2Fms220517a.csv
, there is some time offset in the fades of mark and space with 170 Hz
shift, but not enough to keep one of them out of the noise. I need to run
the experiment with 850 Hz shift.