Lol maybe it's like Harry Potter and Lord Voldemort - 'neither can live while the other survives'...i.e. if one dies, so must the other. Hang on. Actually, this whole HP thing could make sense!! This episode did have slight likenesses to HP - Ben's 'apparation'
also it can be said that Ben resembles Harry and Widmore resembles Lord V...LOL. I'm just going off on one now...
Anyway. yes, it's possible that Charles is Ben's constant. I did wonder why Ben said that he couldn't kill him...does he mean literally?