I remember when I was trying for my daughter, I didn't have soy milk at all, and it was only because I had read somewhere that it was a boy food. I am lactose intolerant so I had a really hard time with all the cow's milk on the girl diet!) I can't remember where I read it now, it was most likely on this site though, as I got all my info from this site.
I did take folic acid. I took the recommended amount for pregnant mums, which I think was 400mcg? I don't remember reading anywhere about taking higher amounts of folic acid for twins, but I don't think I had found this site when I was trying for my twins.
Make sure you eat the yams with the skin on, as that is where all the phyto-estrogens are. That is the key - the phyto-estrogens. I think edamame beans may be full of phyto-estrogens too?
My husband ate the yams too! that wasn't really necessary but it was good for moral support! and neither of us have eaten a yam since Nov 13th 2006 - when we found out I was pregnant! We totally sickened ourselves of them! but it was worth it!
Good luck and I will be sending you lots of twin vibes!!