My common law spouse and I have been together for 6 years. He is the biological father of our twins. However, I did not include his name in the birthcertificate. We are now planning to get married. How do I go about putting his name on their birth certificate?

First this is a legal question so has been moved to the appropriate forum.

Second, any question of law needs to include you general area as laws vary by area.

If the father acknowledges paternity this could be very simple, just needing to apply to a court for an amendment of the birth certificate. Or it could become more complex with questions about why you falsified the birth certificate by not listing the father.

What I can tell you is, if he's the biological father then adoption is not an issue. He needs to be acknowledged as the legal father. How that is done, depends on your area.