Are you talking about the Postgraduate Masters Loan? There is no maintenance loan for that, it's just a straight payment of £12,000 split across your course which is supposed to cover both fees and maintenance. Unless you're eligible for a means-tested bursary, you can't get any additional maintenance money.
Are you planning to do medicine as a graduate entry accelerated course, or just as a straight 5-year undergrad degree? You ordinarily can't get a student loan for a second undergrad degree (maybe medicine is an exception). If you do postgrad medicine though, then the NHS pays your fees in the second, third and fourth years, and you can claim a small maintenance grant. But you have to pay the first year yourself.