How to make common fields of two form share the column in same Infopath Form Library

1- Create two forms with common fields in design view. Example Form1 with field txta, txtb and Form2 with field txta , txtc.
2- Publish both of them as Site Content Type on the site and store the form templates in a document library. This step will also create the site columns specific to items within the form.
3- Create a form library which needs to share the columns between two forms. Allow management of content types for this form library.
4- Attach both Forms as content type in this form library.
5- Now publish the second form (Form2) to this existing form library as template to the document library. It will wipe out the Form1 specific columns (txtb) from the first content type form.
6- Now add the missing column from existing site columns.
7- Give the proper reference of first content type from existing document library.