Nontrivial capability graphs with mistrusting parties

Hi all, 

I would like to create relatively complex communication graphs of in 
face of multiple mistrusting parties (edges are capabilities, nodes are 
capability objects).

For example, both A and B have secrets on which they want to set up a 
joint computation in such a way that none of them can access results of 
a joint computation without being accounted and declassified by another 
party first. In other words, they want to construct an object graph 
like:  

 Secret A -\                      / - Accounting/declass B - A
            - Joint Computation -
 Secret B -/                      \ - Accounting/declass A - B

One might say that there is a trusted third party that creates such 
graph, but maybe there is a more elegant solution that doesn't require a 
third party? 

Thank you,
Anton