test for default graph with overlapping constituents

Do we have a test involving overlap between constituent graphs of a 
default graph? Please send me a pointer. Thanks. - Souri.

Graph G1:
<http://John> <http://ssn> "111223333"^^xsd:integer .
<http://John> <http://spouseOf> <http://Mary> .

Graph G2:
<http://John> <http://ssn> "111223333"^^xsd:integer .
<http://John> <http://spouseOf> <http://Maria> .

If default graph is made up of the above two graphs, the test should 
show the result of a query:

FROM <G1.nt>
FROM <G2.nt>

select ?x ?p ?o
where {?x ?p ?o .}