You could convert the 1st and every second comma then on into another character such as * using the 'mid' function and 'mid' statement.
Then split it.
Then loop through the array and replace the * again.
You could leave the comma out of the value in the first place.
And add a comma to the space with 'replace' later if required
<select name="mySelect" multiple etc>
<option value="Jones John">Jones, John</option>
<option value="Morris Mike">Morris, Mike</option>
If the first and last names have come from a DB I would look into using the index of the record as the select inputs value attribute and working arround it that way.
Depends on what you wish to do with the values next.
They say, best men are moulded out of faults,
And, for the most, become much more the better
For being a little bad.