The birth records list them as twins, but they're only labeled as "first" and "second," and the name fields are blank. How do I index these without them showing up as duplicates?
You are indexing, not tree building, so don't worry about how they show up.
For tree builders: Were they stillborn? If not, building out their individual profiles will quench the duplicate hints. Otherwise, let them show up as duplicates; dismiss the hints; and add a Note to each profile, explaining that they are twins.