If the only reason you would be adding your girlfriend to your policy is to save money, I wouldn't recommend it. At the end of the day, your insurance company would be insuring an additional person and vehicle so regardless of the multi-person or multi-vehicle discount you would receive, it probably wouldn't offset the additional insurable interests.
There's a couple of things you should consider with renters insurance. First, it not only covers your personal property but also your liability.
Here is a good article on sharing renters insurance with a roommate. In my personal opinion, sharing a policy with any person who isn't a spouse is not a good idea.
The vehicle will need to be insured at the address where it stays the majority of the time. Also in Florida, since registered vehicles must be insured by the person who owns them, your girlfriend will probably need to be listed as an additional driver on your policy before a company will offer coverage.