check if one of two fields are null

[Nathan Maves]

I need to have a dynamic query that appends a where clause if one of  
two fields are null.

I could only come up with
<isNull property="field1">
        DO THIS
</isNull>

<isNull property="field2">
        DO THIS
</isNull>

This is obviously wrong since the code could be appended twice.

I need to combine these two statement so that DO THIS is only  
executed if one of the fields (field1,field2) are null.

Nathan