2 Daemon Princes should be able to kill it in 1 turn on the charge. It can't use it's shield in CC and if they Smash, they have between them 8 attacks with rerolls to armour penetration. On average this would give you 4 penetrating hits and 1 glancing. So you have 4 attempts to roll a 5+ on the damage table, and if you can do that, the knight goes down.
And even so, if I remember correctly, Knights only have a WS of 4, so with your princes WS 9, it's still far from guaranteed it will kill a prince should it survive.
My understanding is that they are wounds to a model, not to a unit. So if he only has 4 attacks he can only remove 4 models.
Also this. Plaguebearers shouldn't have too many problems tarpitting one of those fuckers.