Is the American Revolution Legitimate, and if it was, is it possible for the for it to be legitamate and the southern secession not?

In the end, for practical purposes, the American Revolution was legitimate because the rebels won, and the Southern succession was not because they lost. The moral justification for each (which I suspect is what you are referring to) is more complicated. The South was represented in government in a way that the colonies were not, but still faced a certain measure of control from outside. Weather that justifies revolution is another question, however…

In addition:
Perhaps because the southern states had a voice in government and were represented in congress wheras the colonies had no members of parliment to their credit. The south participated in government, had its say, and was bound to live by the laws that the government established. Getting upset and refusing to play with the other kids because things dont go your way is something that is acceptable in small children, not in entire regions of the nation.

The American colonies on the other hand had no voice in the government that ruled over them and therefore had no recourse but revolt.

I think before each of the wars, it was legitimate, and after, only the revolution was legitimate.

Its the victory factor that seperates them.