Why would you want to?
Essentially, you're asking to multiclass two forms of a paladin. These forms are like night and day. Cavaliers gravitate towards good. Blackguards gravitate towards evil. In a sense, it would be like a warlock having two different pacts (i.e. infernal and fey).
I guess I'm having difficulty seeing how a character can represent both a virtue and a vice.
Hear hear.