Chimeric reporter-plasmids, where the expression of a reporter-gene is regulated by a promoter from a different gene, are used to study the regulation of gene transcription. This is done by transiently transfecting cells in which the "experimental" promoter is active with the chimeric reporter-plasmids, and measuring the biological activity of the reporter-protein. When performing these experiments, the cells must also be transfected with a second chimeric reporter-plasmid, there expression of different reporter-gene is regulated by different promoter, usually a strain viral promoter. Why is co-transfection of two promoter-reporter plasmids always required when conducting these kinds of experiments? Briefly, describe how this is done.