We are in agreement now. The index I used for the 1st Georgia Cavalry listed this man under a different last name. Thomas J. Dobbs enlisted at Carrollton GA, March 21, 1862, a reident of Polk County GA, serving from March 1862 through November of 1864.

Since a man cannot be in two different places at the same time, the man who lost his arm at Sharpsburg has to be someone else. I can't think of a reason why you might not be related to both soldiers, but these are two different men.