how can we merge two java objects in one xml file?

Giving a precise answer without the implementation and annotation of your Employee class is hard to do.

Therefore I wrote a small example which is highly related to yours. I hope this is helpful :)

Hint: The important part is writing the XML-Annotations.

Main.class

import java.io.FileOutputStream;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;

public class Main {
    public static void main(String[] args) throws Exception {
        JAXBContext contextObj = JAXBContext.newInstance(Employees.class);

        Marshaller marshallerObj = contextObj.createMarshaller();
        marshallerObj.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        try {
            Employees employees = new Employees();
            Employee emp1 = new Employee(1, "Vimal Jaiswal", 50000);
            Employee emp2 = new Employee(2, "Kamal", 40000);

            employees.getEmployees().add(emp1);
            employees.getEmployees().add(emp2);
            marshallerObj.marshal(employees, new FileOutputStream(
                    "W:\\employee.xml"));

        } catch (JAXBException e) {
            e.printStackTrace();
        }
    }
}

Employees.class

import java.util.ArrayList;
import java.util.List;

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Employees {
    private List<Employee> employees;

    public Employees() {
        employees = new ArrayList<Employee>();
    }

    @XmlElement
    public List<Employee> getEmployees() {
        return employees;
    }

    public void setEmployees(List<Employee> employees) {
        this.employees = employees;
    }
}

Employee.class

import javax.xml.bind.annotation.XmlElement;

public class Employee {
    private int id;
    private String name;
    private int salary;

    public Employee(int id, String name, int salary) {
        this.id = id;
        this.name = name;
        this.salary = salary;
    }

    @XmlElement
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    @XmlElement
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @XmlElement
    public int getSalary() {
        return salary;
    }

    public void setSalary(int salary) {
        this.salary = salary;
    }
}

The resulting XML:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<employees>
    <employees>
        <id>1</id>
        <name>Vimal Jaiswal</name>
        <salary>50000</salary>
    </employees>
    <employees>
        <id>2</id>
        <name>Kamal</name>
        <salary>40000</salary>
    </employees>
</employees>

You should use try-with-resources to close the FileStream object:

try(FileOutputStream os = new FileOutputStream("c:\\temp\\employee.xml")){
 ...

 marshallerObj.marshal(employees, os);  

}

Create a class with list of Employee objects

public class EmployeeList {

private List<Employee> list;

public EmployeeList(){
    list = new ArrayList<Employee>();
}

public void add(Employee e){
    list.add(e);
}

}

Then serialize this

XStream xstream = new XStream();
xstream.alias("employee", Employee.class);
xstream.alias("employees", EmployeeList.class);
xstream.addImplicitCollection(EmployeeList.class, "list");

EmployeeList list = new EmployeeList();
list.add(new Employee(1,"Vimal Jaiswal",50000));
list.add(new Employee(2,"Kamal",40000));

String xml = xstream.toXML(list);

I hope this will help.