render array with two fields

I have an mjoin with a one to many situation. I want to return a comma seperated list of First and Last Name. So, basically I am needing to merge these two:

            ,{
                data: "CaseDefendants",
                render:  {
                    _: '[, ].DefendantFirstName'
                }
            }

and

            {
                data: null,
                title: "Defendants",
                render: function (data, type, row) {
                    return row.DefendantFirstName+ ' ' + row.DefendantLastName;
                }
            },

but i can't figure out the correct syntax.