A soldier could (but was not supposed to) be on roll in different units at the same time. It's a bit more likely here because one of these two commands organized under authority of the Confederate government, and the other under authority of the State of Georgia.

I would prefer to side-step opinion as to why any one man belonged to the home guard. We really can't speculate about motivation without something to base it on. I will mention an episode in which county militia came out to oppose Wilson's Raid in April 1865. Confederate Captain Charles T. Hardman, 6th Alabama Cavalry, witnessed a militia stampede brought about by a report that Wilson's men were advancing upon their camp. The "Yankees" turned out to be a column of slaves being moved by a plantation owner to a point out of reach of the enemy. Hardman wrote home about these "trifling cowards, hallowing for their horses. They want to see fighting done, but are unwilling to do it themselves."

Charles Frazier, the author of Cold Mountain, wrote about his ancestor, the soldier named Inman. According to oral history, Inman was killed near his home by Teague's Home Guards.
http://www.archives.gov/publications/prologue/2004/summer/inman.html

We can't characterize all Home Guards are behaving like Teague and his men, but some did. Several years ago I presented a study of Home Guard activity in Shelby County, Ala., which involved my wife's ancestors. The situation involved Home Guards, deserters and at least one man who never joined either side but represented himself as a Confederate recruiting officer. With the help of the Home Guard, the faux Confederate killed at least a dozen men in Shelby County during the winter of 1864-65, probably more.