relevant between forms

Hi all,

How to express relevant between forms?
For example:

<xform:xform id = "form1">
    ...
    <xform:instance>
        <operator class = "1">
            ...
        </operator>
        <operator class = "2">
            ...
        </operator>
    </xform:instance>   
    ...
</xform:xform>
<xform:xform id = "form2">
    ...
    <xform:instance>
        <enableMachine>true</enableMachine>
    </xform:instance>
    ...
</xform:xform>

...
<xform:selectBoolean id = "sltCtrl" ref = "enableMachine" xform = "form2">
...
</xform:selectBoolean>

How to write the model item of form2 if I want to make control sltCtrl readOnly when 
form1's current operator belongs to class "1" (@class == "1")?

Thanks.

hlyu