The question of the use of one or two rank cavalry formations was a huge bone of contention on several ACW cavalry re-enacting forma about fifteen years ago. The following is a summation of the conclusions reached after several months of ongoing discussions.
Prior to the outbreak of the Civil War the standard cavalry manual was J. R. Poinsett's Cavalry Tactics which was a translation of the manual used by the French. It was first published in 1841. Poinsett stipulated the use of a two rank formation. In October of 1861 the US Army ordered the adoption of Philip St. George Cooke's Cavalry Tactics, Or, Regulations for the Instruction, Formations, and Movements of the Cavalry of the Army and Volunteers of the United States as the standard manual. In contrast to Poinsett's Tactics Cooke advocated the use of a single rank for cavalry.
This change was then rescinded by McClellan and the cavalry was ordered back to Poinsett's manual in the early spring of 1862. For the most part, the Army of the Potomac then used the two rank formation. However, there were exceptions. The 1st Maine Cavalry refused to use Poinsett and stayed with Cooke's Tactics until the end of the war. Custer's Michigan Brigade (1st, 5th, 6th, 7th Michigan) also used Cooke until they were ordered to change to Poinsett in the winter of 1863-64.
The Army of Northern Virginia seems to also have used a two rank formation, based on manuals by either Poinsett, George Patten or Lucius Davis.
In contrast cavalry in the west seems to have predominately used a single rank formation. This was the case with much of the Federal cavalry, which only was forced to adopt Poinsett by Wilson just before his raid into Alabama in early 1865. One exception was Minty's Brigade in the Army of the Cumberland which used Poinsett from the start.
Confederate cavalry also fought in a single rank. Wheeler published a plagiarized version of Cooke which was used by his cavalry. Morgan adopted Dabney Maury's Tactics for Mounted Rifles. Forrest is also stated to have used a single rank in combat but the manual he used for training is not known.
Cavalry in the Trans-Mississippi is even more poorly documented. But in the great charge at Marais des Cygnes on October 25, 1864 Benteen's Brigade, charging in a column of regiments each in a single line, broke Marmaduke's and Fagan's line. However, just to be contrary, the 4th Texas Cavalry (later the 12th Texas Cavalry) from Parson's Cavalry Brigade used Poinsett's until very late in the war.
It could therefore be argued that on the game table cavalry in the east should be mounted on stands half the frontage of cavalry in the west.