No other officers in the Civil War mastered the science of the command of Army Groups, as opposed to Armies. Meaning they could both bring forces to bear in numbers no one on either side could match. If officers like Rosecrans or Canby had the command skills of these men, the Deep South States would have fallen during Sherman's March to the Sea.
Grant was better able to recover than Sherman when things turned to poo, and was at least willing to admit error ONCE in his life (Cold Harbor). That was more than Sherman ever did (Kenesaw Mountain).
With the exception of Thomas, no other officers of the Civil War had the chance to command Army Groups. Sherman and Grant were only two of the three men to ever command more than one Army in the field at one single time - and Thomas's only got the chance towards the end of the war and proved to be effective in that role.
Scott, McClellan and Halleck all were the heads of the Army but they never commanded more than one Army in the field during that war and Scott never took direct command, Rosecrans, Pope, Burnside, Hooker, Butler, Banks, McDowell, Lyon, Curtis, Steele, Schofield, Buell, McPherson, Howard, Logan, Sheridan, Meade or anyone else in the Union never had the opportunity to command more than one army.
On the Confederate side of things, Joe Johnston comes closest to being an Army Group Commander because he commanded the Army of the Shenandoah and the Army of the Potomac at Manassas, in the Atlanta Campaign he commanded the Army of Tennessee and the Army of Mississippi and at Bentonville he commanded the Army of Tennessee and the Army of the South, but in the cases the "armies" he commanded were little more than corps or division which then became perminant parts of one army so even in this case it cannot really be argued that he was an Army Group Commander.
Lee was head of the Army and often sent a corps or so into the Shenandoah and he was the commanding officer of Beauregard's task force in the Bermuda Hundred but he never commanded more than one army at a time, Bragg, Beauregard, Hood, Taylor, Kirby Smith, Buckner, Early, Sidney Johnston, Pemberton, Breckenridge, Van Dorn, Price, McCulloch, Polk and Hardee never commanded more than an Army and some of them only had temporary command when the did so.
Therefore, saying that Grant and Sherman were the only men to "master" Army Group Command during the Civil War is kind of mis-leading since it is only a group of three men than can be consider for that bracket and the third man - Thomas - was not noticably worse in the role than the other two.