...nor any State be formed by the Junction of two or more States, or parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
Your quote made it sound like it can't be done, but it clearly can.
Because the bolded condition clearly applies to the second clause only. It refers to States, plural, where in the first clause only a single State would be involved. You might also note the construction the Framers used when the consent requirement applied to multiple clauses was to begin with the phrase, "No State shall, without consent of Congress" and then continue with, "do A, or B, or C," utilizing commas (rather than semicolons) to separate the items.
ML/NJ