Don't.
If you get information in separated fields, store them in separated fields.
Otherwise, you are throwing information away, and making assumptions about the information!
Store them in separate fields, and you can retrieve the "first name" for friendly greetings:
string greet = "Hi " + firstName;
Or add an honorific:
string greet = "Dear Mr " + lastName;
You can retrieve the "full name" from the DB at any time:
SELECT FirstName + ' ' + LastName AS [Full Name] FROM MyTable