Two (or more) characters were once good, but they have now both become bad guys and are enemies on opposing sides. This usually only happens in settings where there is more than one evil faction, although having one or both being evil independently is not unheard of.
The initial break-up is often because of one friend betraying the other, the second friend becoming disillusioned because of this. It can also happen because of differences in ideology or forced separation. The break-up doesn't necessarily have to be the reason for each character's Face-Heel Turn
but often is.
(Do we have anything like this already? I could have sworn TV Tropes does, but I can't find exactly what I'm looking for. Evil Former Friend comes close, but it has one side of the duo staying on the good side whereas I'm looking for both friends turning to the Dark Side but still becoming enemies to each other.)
The best example I can think of at the moment would be Big Boss (Naked Snake) vs. Zero, from Metal Gear.
Needs More Examples
. Needs a Better Description
. Up for Grabs