@XmlType
@XmlAccessorType( XmlAccessType.FIELD )
public class First
{
    private String attr1;
    private String attr2;

    /**
     * @return the attr1
     */
    public String getAttr1()
    {
        return attr1;
    }

    //the other getters and setters

}

@XmlType
@XmlAccessorType( XmlAccessType.FIELD )
public class Second
{
    private String attr3;
    private String attr4;

    //getters and setters

}

@XmlRootElement
@XmlAccessorType( XmlAccessType.FIELD )
public class HeadElement
{
    private First  firstElement;
    private Second secondElement;

    //getters and setters
}