Well, the Articles of Confederation were pretty unworkable, and almost impossible to amend significantly. If the Constitutional Convention fails, and it's pretty easy for that to happen given the differing demands of large and small states, federalists vs states rights types, etc. If e.g. Rhode Island insists on the AoC amending formula (unanimity) for the acceptan e of the new Constitution (and the Convention's mandate was to Amend, not discard the AoC), they would have held the Convention hostage for concessions that the bigger states wouldnt be able to swallow.
At some point, you will get two states going to war, and then the whole United part of the US goes out the window. I'd expect there to end up being about 5 confederations, rather than two, though.