In my opinion there is absolutely no need or point in having a multiples section in the pregnancy area. Its perfect in the parenting area so you get experienced twin mums visiting often who are THE best people to turn to for advice. There may be a few non pregnant twin mums browsing the trimester forums (like I do) but mostly, unless they're pregnant too, probably not so many. So you've got mostly the wrong audience.
Having been pregnant with a singleton before my twins, they just don't compare. The pregnancy bit is mainly full of singleton mummies and while I'm not saying they don't have good advice to give expecting multiple mums, they definitely won't be as useful in an experienced sense.
I'm always looking through the trimesters and regularly pointing expectant twin mummies to the twin and multiples section, should they need or want specific twin related help or just company.