Shared composition of TDB datasets

Hello Jena community,

I'm looking for a possibility to SPARQL two different TDB backed
datasets within one query. The background is that I have a few TDB
stores (say A and B) and one special TDB store S containing a few
triples that I want to "share" with the other stores.
For instance I want to do something like this:

SELECT ?o WHERE { GRAPH <S> { ?s ?p ?o. } GRAPH <A> { ?s1 ?p1 ?o.} }

In addition, S needs to be writeable and the changes need to be promptly
visible for other queries. Even if I change A to B in the above example.

I know about named graphs but unfortunately they are no option in our
scenario. Could I do it by using assemblers? Or is there any other
possibility?

We use the current versions of Jena/ARQ/TDB in pure Java.

Thanks in advance
Andr?