It is consistent with the dual sovereignty thesis to say that "what is delegated can be withdrawn" but the entity doing the "withdrawing" is the people (by some democratic process, presumably, e.g. a new constitutional convention), not the states, for it is the people who did the delegation in the first place.
So, if the people (legal voters in the states which planned to withdraw) had called a constitutional convention, whose vote was for withdrawl, it might (in your opinion) been a legitimate undertaking with binding result? I don't think the North would have accepted any withdrawl, not matter how it was decided within the South. The Feds, and indeed any government, tends to strongly oppose any move which lessens its authority. It is a credit to the USSR that it was able to allow even those satellite states, forced into survitude, to peacefully withdraw. --Steve