Can anyone possibly enlighten me as to why when reciting the first verse of Shema one covers their eyes using one hand (the right hand) as opposed to when lighting Shabbos candles one is to cover their eyes with both hands? Any insight would be greatly appreciated.
Although I don't have an exact source for an answer, a closer look at the 2 cases can provide a possible answer.
The reasons for the eye covering are different for each case.
By Shema it's in order to keep your concentration focused. Although the Shulchan Aruch says to cover your eyes with your hands, the Mishna Berura (65:17) says it means your right hand. The Shaare Teshuva brings some who say differently. However, since it's helping to do a mitzvah, the minhag became to use the right hand since the right hand is considered more chashuv for a mitzvah. (Chabad.org)
By candle lighting the point is to cover the candles so that you don't have benefit from the candles until after the bracha. The language of Shulchan Aruch (263:5) is that you use your hand to cover the candles not your eyes. Since the minimum amount of candles to light is 2, it was more prudent to use both hands. Over time it became simpler to just cover your eyes, but since it started with 2 hands they continued using both.