I have been using the Duet system from Novagen (or whatever it is called these days), specifically the pETDuet-1 and pRSFDuet-1. Co-expression of my proteins did not work in either vector. Either, one protein expressed or the other. I played around with the promotors (they are both T7) by changing one to the tac promotor. This increased the expression of this gene but shut off expression of the other. The only way I could get my proteins to co-express was to use pGEX vector with one protein, and pRSFDuet with the other protein (leaving the second MCS empty). There is a paper which sums up co-expression in E coli.
http://www.nature.com/nmeth/journal/v3/n1/full/nmeth0106-55.html Dan
