Put one attribute out of two or nothing XSD schema

I'm working on a XSD schema and I'm facing this problem, I want an element X to have only one attribute out of the two possible or no attribute at all for example

<a Attribute1="something"/> --> validates
<a attribute2="another thing"/> --> validates
<a attribute1="something" element2="another thing"/> --> validation failed.
<a/> --> validates

I tried something but it allows the two attributes and that's what a want to avoid

<xsd:element name ="a">
<xsd:complexType>
    <xsd:attribute name ="attribute1" type="xsd:string" use="optional"/>
    <xsd:attribute name ="attribute2" type="xsd:string" use="optional"/>
</xsd:complexType>  
</xsd:element>

I also tried this piece of code put it doesn't validate a 0 attribute tag

<xsd:key name="key">
            <xsd:selector xpath="."/>
            <xsd:field xpath="@attribute1|@attribute2"/>
        </xsd:key>  

anybody with a solution or an idea?