Hi hun. I'm sure that this is frustrating for you. I've got a bit of information for you to think on, and discuss with your own doctor.
First, Marvelon is a low dose birth control pill with the hormones estrogen and progestin. Because it's low dose, it may not be the strength that you require, and because it contains progestin, your body may be reacting to that, which could suggest a need for another kind. Don't worry about that though, because many times it takes a couple of tries to find the right pill.
Now, the other thing I wanted to discuss with you is that because it's a low dose bc pill, it is even more important, that you take it at the same time every single day. All birth control pills require taking them during the same time frame, give or take a few minutes or an hour. However, with low dose, when you are a few hours late one day, and an hour late the next day, or a few days later, etc., your body gets confused about the amount of hormones being introduced, and you'll begin to see some inbetween period spotting. Despite what some of the posters above are saying, it is not normal to have 2 periods per month while on birth control, unless the birth control is a depo provera injection, or a 24/7/31 days per month pill. Yours is neither of those things. Your pills are meant to give a 7 day break for the period to begin and end. If you are experiencing spotting, the spotting can become heavier if you're really off on your schedule for more than a day or two. You're also much more likely to become pregnant during this time, even if you are bleeding, because you'll still be ovulating for your regular period. For some women, this can cause a double ovulation as well, and that makes it twice as easy to become pregnant. There is more involved with double ovulation, of course, including hereditary factors, but it does happen. So this is something to be very aware of at the moment.
Being that you are bleeding twice a month on your Marvelon, I would suggest that you either refrain from sexual activity until you see your gyno and determine the next move, or use condoms as a back up. Your pills are clearly not working as they were intended to work, so you are at high risk for pregnancy right now.
There are many other things that could be causing these double periods, such as ovarian cysts, a tear in your uterus, or some other internal injury. The only way to find out the problem, is to make a dr's appointment. Between now and your appointment time, make some notes about how regular you were in taking the Marvelon, any injuries to the abdominal area you may have received, any pain you are having, or illness you have experienced in the last couple of months.
Also, is it possible that you could have become pregnant (it is if you were not regularly taking your pill on time), and have suffered a miscarriage ? Did you experience an extremely heavy period with some unusual cramping over the last two months, and have been spotting and/or having these two periods a month since then? If your answer is yes, then please be sure to discuss this with your gyno. You may require a d&c to clean out your uterus, so you won't get an infection. This is very important so please don't overlook it. If you feel sick or feverish, do not hestitate to get into your gyno that same day or go to an ER to be checked for infection.
More than likely you're just having some heavy spotting due to not taking your pills on time, so don't worry to much, but do make that appointment, ok?