Help with STRING database data
Proteins evolve and have different sequences between species, so you would have to define what you mean with "same protein". One option would be to use an orthology database like eggNOG. (eggNOG has the same protein identifiers as STRING.) Then you could figure out 1:1 correspondences between proteins. 

You probably also want to read up on Roded Sharan's work, e.g. Global alignment of protein-protein interaction networks.