What a settable and gettable computed function should look like

gistfile1.js

firstName: SC.Record.attr(String),
lastName: SC.Record.attr(String),

fullName: function(key, newValue)
{
  if(newValue) {
    var names = newValue.split(" ");
    this.set('firstName', names[0]);
    this.set('lastName', names[1]);
    return this;
  }
  return this.get('firstName')+" "+this.get('lastName');
}.property('firstName','lastName').cacheable()