James Hopkinson, a former instructor at the Infantry Battle School in Brecon, has described the challenging conditions that the two soldiers who died would have been up against.
He said that military exercises in the Brecon Beacons were designed to push soldiers "to the edge of their endurance," but that measures would "undoubtedly" have been put in place to mitigate the effects of the hot weather.
He added that while the exact circumstances of the deaths remain unclear, such endurance training is needed to make the British Army "the best in the world".
Emotional tributes have been paid to a third army reserve soldier who died from injuries sustained during SAS selection training.
One of two Army reservists killed in a military exercise in the Brecon Beacons on Saturday has been named as Lance Corporal Craig Roberts.