hi PKriyshnA.
have you tried this:
<xs:element name="employee">
<xs:complexType>
<xs:all minOccurs="0">
<xs:element name="FirstN" type="xs:string"/>
<xs:element name="LastN" type="xs:string"/>
</xs:all>
</xs:complexType>
</xs:element>
The example above indicates that the "firstname" and the "lastname" elements can appear in any order and each element CAN appear zero or one time!