XSD choice of sequences

I want an element <aaa> to contain either a sequence of element <bbb> or a sequence of two elements <bbb>,<ccc>. For example, the following is valid:

<aaa>
  <bbb>
  <bbb>
</aaa>

and

<aaa>
  <bbb>
  <ccc>
  <bbb>
  <ccc>
</aaa>

but the following is invalid:

<aaa>
  <bbb>
  <ccc>
  <bbb>
</aaa>

I have tried the following scheme

<xs:complexType name="aaa">
  <xs:choice minOccurs="0">
        <xs:sequence  minOccurs="0" maxOccurs="15">             
            <xs:element name="bbb" type="xxx" /> 
            <xs:element name="ccc" type="xxx"/>
        </xs:sequence> 
        <xs:sequence  minOccurs="0" maxOccurs="15">
            <xs:element name="bbb" type="xxx" />
        </xs:sequence>
  </xs:choice>
</xs:complexType>

but I have this error message when validating with JAXP: cos-nonambig: bbb and bbb (or elements from their substitution group) violate "Unique Particle Attribution" Is there a way that I can obtain my goal? Thanks in advance