The town with ferry access to both MV & Nantucket is Hyannis. You can visit both, however I think it would be exhausting to do both in that short of time period.
Hyline Ferry service out of Hyannis offers a one day ferry schedule that'll take you to Nantucket for an afternoon visit for a few hours and then on to Martha's Vineyard for a few hours more. (lunch on Nantucket and dinner on M.V.?) It's possible to do this in a day although you'll hardly get a 'taste' (even though you're having meals there) of either island.
Travel on the Cape in Sept. is so much easier than it is in the summer, you can really stay in any town and drive to the ferries in Hyannis. (Hyannis is not where I'd suggest a first time visitor stay, however.)
Here's the link for one day ferry trips to both islands:
Falmouth is a great town. MainStreetMan has posted great information here about restaurants, the bike trail, beaches, etc. You'll find all this information by doing a search on this forum.
For more information, just ask...you'll get plenty of help. Have a great time!
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