PREFIX  rdf:    <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX  rdfs:   <http://www.w3.org/2000/01/rdf-schema#>
PREFIX  foaf:   <http://xmlns.com/foaf/0.1/>

SELECT *
{ 
    # Does not work - why?
    GRAPH ?g1 
    { 
        ?x foaf:knows ?y . 
    }

    GRAPH ?g2
    { 
        ?y foaf:knows ?x .    # x and y reversed
    }
}