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I guess, I'd choose matching Names, though I wouldn't start games on initials.
I think, a single letter won't make a matching set of names.
In my opinion, most times it's rather ridiculous.
I know a family, who named their kids like John-Matthew, John-Andrew and John-David (made-up combinations, but theirs were similar).

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I'd go with:
approximately the same length
approximately the same popularity
the same style

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for instance: Theodore and Victoria, Theresa and Rebecca, Jonathan and David, ...