What I failed to consider is the common proclivity towards "matchy" names. What seems to happen quite frequently is for one twin to get a popular name, and the other to get an uncommon "match". Names that are quite uncommon overall are disproportionately more common for a twin.
Now, the data isn't perfect, of course--it only shows most common twinsets. So, a name like Kaylee, which has dozens of spelling in use, doesn't rank highly at all. Also, a name could theoretically be in use frequently, but not paired with any one name particularly often. It wouldn't show up in these rankings either.
So, with those limitations in mind, here're some interesting stats:
The most common twin pairs are: Olivia & Sophia, Madison & Mason; Daniel & David.
However, if you break the pairs up:
| G/G Names | B/G Names | B/B Names | |||||
| Olivia | 142 | Mason | 70 | Jayden | 120 | ||
| Sophia | 142 | Madison | 60 | Logan | 98 | ||
| Emma | 120 | Ethan | 49 | Ethan | 91 | ||
| Isabella | 91 | Emma | 44 | Joshua | 90 | ||
| Ava | 74 | Isabella | 39 | Jacob | 89 | ||
| Abigail | 60 | Olivia | 39 | Isaiah | 85 | ||
| Madison | 58 | Abigail | 38 | Andrew | 69 | ||
| Ella | 57 | Jayden/Jaden | 37 | Alexander | 67 | ||
| Makenzie/Mackenzie | 88 | Jada/Jayda | 36 | Benjamin | 67 | ||
| Gabriella | 48 | Addison | 26 | Nathan | 66 | ||
| Faith | 46 | Aiden | 25 | Matthew | 65 | ||
| Makayla | 40 | Alexander | 25 | Daniel | 58 | ||
| Emily | 34 | Sophia | 24 | David | 58 | ||
| Hope | 33 | Emily | 22 | Jordan | 53 | ||
| Hailey/Haylee | 31 | Chloe | 21 | Noah | 53 | ||
| Heaven | 29 | Jacob | 20 | Elijah | 52 | ||
| Nevaeh | 29 | Ava | 19 | Michael | 52 | ||
| Mia | 26 | Michael | 19 | Isaac | 47 | ||
| Addison | 24 | Andrew | 18 | Nicholas | 46 | ||
| Elizabeth | 22 | Owen | 18 | Aiden/Ayden | 42 | ||
| Hannah | 18 | Zoey | 18 | Evan | 39 | ||
| Avery | 15 | Zachary | 17 | Lucas | 39 | ||
| Arianna | 14 | Isaiah | 14 | Jeremiah | 33 | ||
| Brianna | 14 | Jackson | 14 | Anthony | 31 | ||
| London | 14 | Ella | 12 | Jaylen | 26 | ||
| Paris | 14 | Taylor (g) | 12 | Mason | 26 | ||
| Grace | 13 | Tyler | 12 | Justin | 25 | ||
| Kaylee | 13 | Landon | 11 | Kayden | 25 | ||
| Samantha | 13 | London | 11 | Landon | 25 | ||
| Anna | 12 | Morgan | 11 | William | 24 | ||
| Chloe | 12 | Oliver | 11 | Caleb | 23 | ||
| Evelyn | 12 | Benjamin | 9 | James | 23 | ||
| Reagan | 12 | John | 9 | John | 23 | ||
| Riley | 12 | Mackenzie | 9 | Josiah | 21 | ||
| Zoey | 12 | Matthew | 9 | Luke | 21 | ||
| Sarah | 11 | Michelle | 9 | Chance | 20 | ||
| Jayla | 10 | Adrian | 8 | Chase | 20 | ||
| Katherine | 10 | Adriana | 8 | Hayden | 20 | ||
| Kayla | 10 | Charlotte | 8 | Hunter | 20 | ||
| Morgan | 10 | Elizabeth | 8 | Gabriel | 19 | ||
| Mya | 10 | Henry | 8 | Joseph | 19 | ||
| Natalie | 10 | Isaac | 8 | Samuel | 17 | ||
| Nicole | 10 | Jayla | 8 | Christian | 16 | ||
| Serenity | 10 | Samuel | 8 | Christopher | 16 | ||
| Trinity | 10 | William | 8 | Brandon | 15 | ||
| Autumn | 9 | Blake | 7 | Bryan | 15 | ||
| Sophie | 9 | Brooklyn | 7 | Liam | 15 | ||
| Summer | 9 | Caleb | 7 | Jason | 13 | ||
| Valentina | 9 | Colton | 7 | Maddox | 13 | ||
| Valeria | 9 | Connor | 7 | Garrett | 12 | ||
| Jack | 7 | Gavin | 12 | ||||
| Jordan (b) | 7 | Henry | 12 | ||||
| Natalie | 7 | Jeremy | 12 | ||||
| Nathan | 7 | Jonah | 12 | ||||
| Robert | 7 | ||||||
| Trinity | 7 | ||||||
| Tristan | 7 | ||||||
| Zane | 7 | ||||||
| Austin | 6 |
Most of the names aren't too surprising. Logan, however, is ranked either #20 or #26 (depending on whether you prefer the SSA ranking or the NameNerds rankings). Why is it so much more popular for twins? Because of its matchy-ness with other names--particularly Landon, Lucas, Luke, & Liam. Hope is down in the 200s, but it's commonly paired with Faith. Jayda/Jada is fairly uncommon as well, but is frequently paired with Jayden/Jaden & Jordan.
David & Daniel are the most common twinset for boys...but they only appear paired with each other! There are a few of these sets--Heaven & Nevaeh, London & Paris, Reagan & Riley, Jayla & Kayla, Summer & Autumn, Valentina & Valeria; Blake & Brooklyn, Taylor (g) & Tyler; Chance & Chase, Hayden & Hunter, Christian & Christopher. Unsurprisingly, "matchy" names are more common for g/g sets.
(It's a bit depressing, actually.)
I also find it interesting that of all the unisex names, only Jordan shows up as only male. Jordyn is a fairly popular name, so I'm at loss as to why it doesn't appear in twin-sets often.
