Joseph Gullion and Rebecca Powers were married by John Stanger, who was the minister of Zion Lutheran Church near Speedwell in Wythe Co. , so Rebecca probably lived not far from his area of travel. One of my Shupes, Henry b. 1800, also married a Hester Gullion, but that was in Grayson Co. I have not been able to determine her parents.
In one of my Kegley books I found a John Powers age 14 and Hickman Powers age 13 listed by Overseers of the Poor in 1832 as being the children of Polly Hopkins. This John sounds too old to be the same John Powers who came to WVA, but it sounds like another case of out of wedlock children unless this Polly had remarried. I don't know if any of this is helpful but it gives a little more to research. I think you are on the right track in assuming that both children, George and Joseph, had different fathers who probably lived nearby when the children were born.