1. dubya = w in a Texan accent, now a nickname for George 'Dubya (W) Bush' - US president, apparently
2. bubba (also bub, bubbah) = 'mate', 'mush', 'pal'; way to address a male. From German or Yiddish.
I agree with the Dubya and possibly the history of the second, but Bubba is more a sort of generic nickname for a southern redneck - because so many good ol' boys have that as their nick name. Bubi or Bubby is the yiddish term used as "mate"