Libya
To be exact - and at the risk of being rather sad and spotterish - the primary SR71 for the initial BDA flight was flown by Jerry Glasser and Ron Tabor, with messrs Shul and Watson flying the spare. Both aircraft performed faultlessly, but there was cloud over the target, so they got no usable imagery. The crews swapped roles the following day but the primary had a malfunction of the optical bar camera, and the targets were obscured by sand storms. For the third mission, the primary was flown by Bernie Smith and Denny Whalen, with Shul and Watson as spare. This time the targets were clear and they got their pictures. According to my book, the recce team were frustrated because they could have got the BDA info from the first sortie if they'd been allowed to use their radar sensors.
Apparently the missions were timed so that the primary aircraft would be coming off target as the spare was turning into the Mediterranean refuelling track. If the primary had a problem, the spare would continue to the target. Otherwise it would turn back to England.
For (much) more detail, see pages 290 to 301 of 'Beyond the secret missions'. It mentions Shul heading out to refuel off Cornwall, and seeing the returning F111s several thousand feet below, and that they acknowledged each other by rocking wings