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Have you tried with Unwrap UVW?
You apply the Unwrap UVW modifier to the object whose UV you want to stitch, in the "Edit UVs" section you click the "Open UV editor" section, you move the two UV pieces that you want to stitch together close to each other, then you press either one of the stitch buttons in the "Stitch" menu and they should stitch together.
Hope this helped 🙂