1 To 1 Relationships in SQL are made by merging the field of both table in one !
I know you can split a Table in two entity with a 1 to 1 relation. Most of time you use this because you want to use lazy loading on "heavy field of binary data in a table".
Exemple: You have a table containing pictures with a name column (string), maybe some metadata column, a thumbnail column and the picture itself varbinary(max). In your application, you will certainly display first only the name and the thumbnail in a collection control and then load the "full picture data" only if needed.
If it is what your are looking for. It is something called "table splitting" or "horizontal splitting".
https://visualstudiomagazine.com/articles/2014/09/01/splitting-tables.aspx