I added some tests that illustrate the problem. They can be found in AutofillProfileTest.SaveAdditionalInfo_Name_*.

For example when execution a.SaveAdditionalInfo(b) with a and b having complementary name values, a should have a complete name but instead has only the name parts from b.

Example for 
a: First_name: John
   Last_name : Doe

b: Full_name : John Doe

Expected:
a: First_name: John
   Last_name : Doe
   Full_name : John Doe

Observed:
a: Full_name : John Doe