Found It!

Thanks to some l33t google searching, I found a mathematical equation that gives twin premature survival rates. P(m) = (375 x P(s)) / (275 + P(s)) with P(m) being the survival rate for multiples and P(s) being the survival rate for a singleton pregnancy. As you will see, the rate of survival for multiples is very slightly higher than for singletons even though they may have a smaller birth weight. I will do the math for you and make a chart!

21 weeks or less: 0% single & multiple
22 weeks: 0-10% single / 0-13% multiple
23 weeks: 10-35% single / 13%-42% multiple
24 weeks: 40-70% single / 47%-76% multiple
25 weeks: 50-80% single / 57%-84% multiple
26 weeks: 80-90% single / 84%-92% multiple
27 weeks: > 90% single / > 92% multiple

These are all rounded down to the nearest percent. It is true that there have been babies born at 21 weeks and earlier that have survived, but the rate is so very low that it's not really measurable. This is why I have kept it at 0%. These rates also depend a lot on weight of the baby/babies at birth. A lower weight baby is going to have a far smaller chance of survival than a higher weight baby born at the same gestational age. They've also shown that female babies have a higher rate of survival than male babies born at the same gestational age. How far has science come!