Is it possible to express the cistrons from a polycistronic insertion fragment in a single plasmid?
If I understand correctly, you are seeking option for polycistronic expression. Have you considered using T2A or P2A sequences? These come from virus and essentially break polypeptide into two parts, so-called self-cleaving peptide T2A. Real mechanism of its work is that ribosome stalls on the sequence and unable to create peptide bond, effectively re-starting translation with next after T2A amino acid.

Of course, you lose independent control over expression, both products will be expressed at same level (but degradation might be different).

So, you should consider creating construct that looks something like that:

lac:gene1-T2A-gene2-stop

Thing is that a) it might not work in E.coli and b) it might not be necessary, because primary purpose of T2A is equal expression level of two polypeptides. And, if your 2 genes are big, for bacteria it is easier to handle two separate plasmids, rather than one but much larger.