How do i set up the editor to work with combined fields?

iteraction

Hi,
I am using inline editor on a table that has two fields ( first name, last name) combined in one.
The column data is displayed using

{data: null, render: function(data, type, row) {
return data.first_name + ' ' + data.last_name;
}}

The editor is set up using

{
label: "User first name:",
name: "first_name"
},
{
label: "User last name:",
name: "last_name"
}

In the end, when i click that field i get the error :

Uncaught Unable to automatically determine field from source. Please specify the field name

Can some1 tell me what am i doing wrong?

Edit: i have tried setting a name for the column, without success:

{data: null, render: function(data, type, row) {
return data.first_name + ' ' + data.last_name;
}, name: "username"}

then i set up the editor like this

{
label: "User name:",
name: "username"
}