I am looking for a poem about being a twin that ends something like:
"I was lucky enough to be born with my best friend."  In addition to
that one specific poem, links to sites with poems about twins would be
appreciated.

I wasn't able to find the poem you are looking for, however, I found
this and thought you would like it:

"...People are always telling us how lucky we are that we're twins. I
used to just nod my head and smile, but lately, since I've written
this paper, I have really noticed how lucky I am. Before I think we
both just took it for granted. I can't even begin to imagine what life
would be like without my twin sister. It would be like losing an
important part of myself. I was always fascinated by the quote,
"Friends are like four-leaf clovers; they are hard to find, but you're
lucky to have them." I guess I can say I was born with my four-leaf
clover because my twin is my best friend, and I didn't even have to
look for her! No matter what, no matter where, we would always be
there for each other whenever one of us need someone through school,
through careers, through marriages, through life. I've found that
lucky four-leaf clover, and I'm not going to lose her."
-- Katie Senkoski, grade 8
http://www.skippingstones.org/sample-18.htm

Best regards,
Rainbow

Here is a good site with poems about twins:

http://www.geocities.com/zoeyl_au/TwinsPoetry.html

Best regards,
Rainbow