Just in case nouse is wrong and you're not doing FAA but JAA then disregard his comments about renting. I rented a twin quite happily in the UK with an IMC rating, from the club where I did my rating.
But as others have said - yes, complex is just a sign of in your logbook, multi is a rating in its own right. I found no great problem moving straight from non-complex (PA28/C172) onto complex twins (PA34).