After you have created a person's card for each twin in your family tree, add a ‘Twin fact’ to each individual to designate the relationship.
To add a 'Twin Fact' to someone in your family tree, find the person in the tree and click on his/her personal card.
Then in the ‘Facts’ section in the left-hand side information panel, click on ‘Add Fact’.
In the 'Add New Fact' window select the 'Birth' option for both 'Category' and 'Type'. Now type 'Twin' into the 'Enter a fact type'.
Click 'OK'. You can add the name of the twin in the 'Description' field.
Repeat the process for the other twin.
Privacy note: Adding living relatives to an online family tree requires you to obtain their prior consent. Before adding living relatives below the legal age to the family tree, you must obtain the consent of their parent or guardian.