The friendship between the two men is genuine; but the conviction has grown within British athletic circles that the Canadian, who this year added the world indoor 110 metres hurdles title to the Olympic title he won last summer, has gained more than the Briton from their three-year association. This year, however, the arrangement has been different - largely, but perhaps not entirely, because of McKoy's move to a new home in Linz last year.
Since March, the pair have done just one training session together. They are not even racing each other as often as in the past. Wednesday was only their second meeting this season and what chance there was of a third in tonight's grand prix at Crystal Palace was apparently decreased by a less than huge offer to McKoy from the promoter Andy Norman, who also manages Jackson.
Whether the two will meet again in Stuttgart is open to question. McKoy wants to do the 110m hurdles and the 100m there, but he is unwilling to travel from Linz to Vancouver for the Canadian trials on 1 August. He is currently awaiting a decision on the matter from the Canadian track authorities.
Jackson appears unconcerned by the whole business, although he says he would be disappointed if the Canadian is not in the world championships. He downplays the importance of their training together.
'It wouldn't make any difference if Mark trained with me now because he has the blueprint,' he said. 'He knows exactly what he has to do to improve. When he first came over to Cardiff, the key for him was learning how to hurdle. The most important thing is to have the technical endurance to keep hurdling smoothly all the way, and that's what he didn't have.'
Although Jackson is clearly able to beat McKoy - he did so three times immediately after last year's Olympics, and has now done so twice this season - the suspicion has been that over-familiarity has affected his resolve on the big occasions. 'When we are training I don't consider Mark to be one of the people I will be racing against,' Jackson said on Wednesday. Perhaps that has not been a good thing.
Whatever the case, Jackson, whose winning time in Nice was 13.12sec, is clearly in superb shape after a winter without injury. 'The main thing I have to do now is stay healthy,' he said. 'You are walking on the edge all the time. I want to feel the way I do now on the day of the Stuttgart final. I could run it now.'
Three Britons selected for Stuttgart seek to show fitness tonight - David Grindley, who takes on the Olympic 400m champion, Quincy Watts; Curtis Robb, who runs the 800m after his viral illness; and Steve Backley, recovering from a shoulder injury, who faces the Olympic javelin champion Jan Zelezny in his first competition since taking the bronze in Barcelona.
Matthew Yates and Steve Cram - if his unexpected non-appearance for training last night does not augur ill - will seek the world championship 1500m qualifying time of 3min 36.50sec, while Linford Christie races the world silver medallist, Leroy Burrell.