The word " form " should be singular in that sentence. " together with " is a phrasal preposition.
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MellaKI agree with Welkins-it's a phrasal preposition. When you see "together with" that means something is being added with another-the result is, of course, that you end up with a plural (so "form" is correct).
You can check this by adding the subjects together: "The duplex examination" AND "the clinical examination of the doner site tissue volu