Ta Da!! I am an expert on this topic!
(Just kidding, it happened to me once a few months ago)I posted several pics too. I am not going to look for them, but I am pretty sure some are in the weed nerd thread in subcool's forum. Mine was Jack the Ripper though.
I didn't know mine were twins till they popped through jiffy pellet I think, anyway I kept training the big one away from little one. When the big one became so big the little one would die, I took cuttings off big one and cit the little off as a clone. I put the big one in to my bud room, but it ended up as a male. I am growing the little twin out and I will try to get picture of that tomorrow. It is still pretty small actually. I have one of the females in there too. I will take pics of both tomorrow afternoon. A few people posted and said the little one always dies, so I worked my ass off to keep it alive. I was pretty happy I thought of chopping it and re-rooting it. Fuck-ton of wasted time on plant count if both mine turn out to be male. Oh well, had to try.
A cool thing about mine, is the little glass vile they came in was cracked and a chip was busted out of it when I got the seeds. I joked that they were little monsters in there and that is why the vile was broken, then one was a double-yolker!! I was convinced it was going to be my mother of all mother plants. lol. good luck.