Yes, It's possible for him to have served in both. A number of Tennesseeians who initally joined CS units later had a change of heart and joined Federal Units. Likewise, a number who had chosen not to join up were conscripted by the Conferderate authorities and felt no obligation to stay with the units they were sent to, deserting at the first opertunity. Thomas's Legion, which also had a heavy Cherokee Indian component, served initally in East Tennessee but was ordered to Madison County, North Carolina, in 'late 1862'. I suspect Arcibald probably left the unit then. The 2nd Tennessee Cavalry was in Louisville on Dec.4, 1862, but was shortly after ordered to report to the Army of the Cumberland. They arrived with the army in time to participate in the Battle of Stone's River- fought Dec. 30 1862 to Jan. 3, 1863.