The RNA polymerase sits on a region of DNA about 30-40 base pairs.  I've no idea how much the 'bulk' of the polymerase complex might cause steric hinderance at a second promoter region nearby but at a guess perhaps a 100 bp spacer should do the trick?!  Considering the polymerase will elongate at 40-50 bp per sec the complex may not be in position long enough to have a major effect (assuming equal promoter strength).....with this in mind maybe you could get away with a shorter distance between the two promoters.  In short......I've no real idea.....try it, find out, let everyone else know!!  Good luck.

Thank you Marcus! I will try and share my experience. 

Seems not matter so much. I current use two different linkers between two outwarding promoters:

1, GTATAC TAGT ATCGAT TCGCGA CC

2, GTATCG ATcctca gc ta GCGATCGC GCG ATCGC agctgagg at TCGCG ACC

Both are working well in either insect cell expression or in human cell expression.

see

http://tf5lu9ym5n.scholar.serialssolutions.com/?sid=google&auinit=Z&aulast=Zhang&atitle=Recombinant+expression+and+reconstitution+of+multiprotein+complexes+by+the+USER+cloning+method+in+the+insect+cell-baculovirus+expression+system&id=doi:10.1016/j.ymeth.2015.10.003&title=Methods+(San+Diego,+Calif.)&volume=95&date=2016&spage=13&issn=1046-2023

 ziguo