(My fault, for some reason I was fixated on the recrossing of the Potomac on the retreat with regard to following information.)
From below references, Ewell's men forded the Potomac, but the troops of Longstreet and A. P. Hill recrossed on a pontoon bridge three miles downstream. I have no reference stating that Longstreet himself crossed in company with Lee, but nothing to refute it either. I would have expected the senior generals to be accompanied by their respective staff officers. Below are some relevant citations.
July 13, 3:30 p.m. Lee and staff forded the (Potomac) river. [Cleveland Morning Leader, July 17, 1863, quoting the American, Berlin, MD, July 16, 1863]
July 13. As Hood’s [actually Robertson's] brigade crossed, Lee, astride Traveler, watched as they passed – each soldier bared his head. No salute, no cheer, no word was spoken as they marched by. [Henry Haywood Daniel, 3rd Arkansas, Hood's division]
General Lee mounted on his horse, standing on the Virginia side, watching the last detachments of his troops returning after defeat. [Account of William Whitehead West, E/Cobb’s Legion, McLaws' division]
Found pontoon bridge being laid where road diverges from river south of Williamsport. Gen. Longstreet was up to his waist in the water, working as hard as any of the detail. The rain was still coming down, and a cold wind had set in from the west. [The Spirit Divided, Charles Holt Dobbs, Chaplain, 12th Mississippi, Hill's corps]
July 14, daylight. Crossed Potomac on a pontoon bridge. The river was flooded from recent rains. A portion of Lee’s army crossed by fording up the river at Williamsport. Dead mules and ambulances were floating down as we crossed. [Fighting with the 3rd Georgia, Alfred Zachry, Hill's corps]
July 13/14, night. Recrossed one mile above [upstream from] Williamsport. [Edward Johnson’s O.R., Ewell's corps]
July 14, a little after sunrise. Crossed at Williamsport, deep fording. Longstreet and Hill crossed on pontoon bridges. [Diary of Samuel W. Eaton, 57th North Carolina, Ewell's corps]
July 14, about daylight. Our division waded above Williamsport, the water was about 4 feet deep. We (also) fixed up pontoons and put them across at Falling Waters. [Letter of Surgeon Abram Schultz Miller, 25th Virginia, Johnson's division]
July 14, 4 a.m. Our [Ewell's] corps crossing, wading up to our shoulders. Longstreet and Hill crossed three miles below on pontoons. [Samuel Angus Firebaugh, 10th Virginia, Johnson's division]
July 14. Waded river up to my arms and two hundred yards wide. [Robert H. Depriest, I/2 VA, Johnson's division]