I've read up on this quite a bit, just trying to find hope that a successful pregnancy is even possible after a MC. In everything I've read (I try to steer clear of message board and try to stick to medical journals) fertility is not increased after a MC, nor is the possibility of multiple births.
http://www.nytimes.com/2008/10/14/health/14real.html
I did read a recent article on NIH.gov's website that the likelihood of multiples increased after the age of 35, but so does the risk of MC, especially if you are carrying multiples, so it's a double edged sword.
If you are interested in conceiving twins, I think you're best bet is IVF or at the very least, clomid - which can cause an ovary to produce two or more mature eggs.