Although I cannot find a more specific instruction regarding the 73's (I imagine there could be one somewhere!) it probably is prudent to use two loco's as the class are infamous for overheating particularly if one loco is handling a heavy train. One example that springs to mind is when a 73 was used as a subsitute loco on a Cross Country service that was put on at Guildford, it was on fire by the time it got to Reading Spur Junction! So perhaps GB Railfreight took the decision to double up as if a failure occured they would have the delay bill to deal with. In addition to reduce the wear and tear on the loco's and as previous posts have stated to compensate for the 600hp individual loco power output double headed is no doubt a good idea!
I'd not disagree with any of that, but there is a difference between an company operational decision and an outright ban
Having said that, overheating should not cause a failure of an EDL, if properly maintained and all systems working correctly, as they were designed to cope with HWT
I've also never heard of one catching fire due to HWT
I have, however, had the fire bells / bottles go off because of it and the HWT protection process not operating as it should, but that is a different story