The D63xx/Class 22 were not used that much as double headed pair. You might get the odd couple on a summer Saturday, or on a heavy ballast freight, but otherwise they were mostly used singly. An exception was using them as pilots to Warships (and initially steam locos) on heavy expresses between Newton Abbot and Plymouth, where they got thrashed to some considerable speeds through the dip at Totnes. Apparently they were not as wild a ride through there as a Warship though.
One of their main problems was that North British had gone bankrupt in 1962, pretty much on completion of the last loco, and the design and ancillary items were pretty much theirs, which were now without primary support. This was different to Birmingham RCW, where the same happened, for they had really just been a fitter of standard proprietary parts, whereas NB manufactured a lot themselves (including engine and transmission) or went to oddball suppliers. The basic engine and transmission were less of an issue than all the peripheral parts, fuel pumps, water tanks, all the electrics, etc. Taunton fitters, from time to time confronted with them, found the overall design pretty poor. Ironically one of the problems was the multiple-unit equipment, which of course you did not find was defective until you coupled them up - and then which one was it? The 5-minute solution to that was provide a second crew for the second loco. The competent North British diesel staff who did the original D600 locos (I think the engines for those five were built in Germany as examples), which although slow and heavy worked pretty well, had seen the writing on the wall and gone a couple of years later by the time the D63xx were built.
The D833 North British Warships did not suffer from this to the same extent because many of the drawings, and selection of component suppliers, had been done by Swindon. You have to be a bit of a geek to spot any external difference between the two Warship versions, so everyone looked at the number first, although inside the engine room the main units are different. North British got the engine drawings from MAN in Germany, but they were of course metric drawings and although the drawing office staff were reasonably numerate with the conversions, this did not extent to the assembly shop staff.