I agree with VickiSoCal. We did similar once, combining Nevis and Antigua, figuring we had 12 days and it was so far to go we might as well do 2 islands. Wasn't worth it to spend an entire day traveling between hotels and planes.
Not to mention that, in this case, I'm pretty sure there's only a single morning flight from Miami to Antigua, so you'd have to spend the night in Miami anyhow.