XML Schema (XSD): How to express "At most one element from group A and at most one element from group b"?

Question

Say I have two sets of XML elements:

<One/>
<Two/>
<Three/>

And

<A/>
<B/>
<C/>

And I want to have them as children of a bucket element:

<Bucket>
   <A/>
   <One/>
</Bucket>

Or

<Bucket>
   <C/>
   <Two/>
</Bucket>

But I don't want to allow more than one element from either set of elements. I.e:

<Bucket>
   <A/>
   <B/>
   <One/>
</Bucket>

and

<Bucket>
   <A/>
   <One/>
   <Two/>
</Bucket>

would be invalid. How might I express this in my XML Schema?

I thought to try xs:unique but that prevents name() and local-name() usage in the field or selector.

UPDATE

The full solution is:

<xs:element name="Bucket">
  <xs:complexType>
    <xs:sequence>
      <xs:choice minOccurs="0">
        <xs:element name="A"/>
        <xs:element name="B"/>
        <xs:element name="C"/>
      </xs:choice>
      <xs:choice minOccurs="0">
        <xs:element name="One"/>
        <xs:element name="Two"/>
        <xs:element name="Three"/>
      </xs:choice>
    </xs:sequence>
  </xs:complexType>
</xs:element>

Answer 1

I'd go with something like,

<xs:element name="Bucket">
  <xs:complexType>
    <xs:sequence>
      <xs:choice>
        <xs:element name="A"/>
        <xs:element name="B"/>
        <xs:element name="C"/>
      </xs:choice>
      <xs:choice>
        <xs:element name="One"/>
        <xs:element name="Two"/>
        <xs:element name="Three"/>
      </xs:choice>
    </xs:sequence>
  </xs:complexType>
</xs:element>

But probably with better types defined to represent the choices instead of having them inline like that. But the general idea is the same.

By default, minOccurs and maxOccurs are set to 1, so by default this doesn't permit multiples of each type to be present. It also means they aren't optional. If you want either group to be optional, you should add minOccurs='0' to the <xs:choice> element.