No need to get snarky, we are not mind readers, we don't know what others know unless they care to share it. Rebecca DID tell you how to do it, a custom query or php routine to sort by a value fragment.
First you'll need to develop a regexp that reliably returns last names from the combined value. Then you can either include it in an ORDERBY mysql statement or use it to build a structured array using one of the array sort functions like asort, ksort, usort, uksort, etc.
From those results, you can easily build your own dropdown.