Follow these steps:
Great grandmother1:
Add parents if not already there.
edit/add the details for these parents as needed.
Great grandmother 2
Add parents to great grandmother2.
Select great grandmother2.
Click on the [Change or delete] button.
Click on the [Change parents] button.
Click on the [Change parents and type] button.
Using the "Set mother" and "Set father" drop downs you can now find and select great grandmother1's parents to be the parents of great grandmother2
Finally click on the [Set parents and type] to save the change.