Promoters can be very closely located in the DNA. Such "closely spaced promoters" have been observed in the DNAs of all life forms, from humans to prokaryotes and are highly conserved. Therefore, they may provide some (presently unknown) advantages.
These pairs of promoters can be positioned in divergent, tandem, and convergent directions. They can also be regulated by transcription factors and differ in various features, such as the nucleotide distance between them, the two promoter strengths, etc.
The most important aspect of two closely spaced promoters is that they will, most likely, interfere with each other. Several studies have explored this using both analytical and stochastic models. There are also studies that measured gene expression in synthetic genes or from one to a few genes controlled by bidirectional promoters.
More recently, one study measured most genes controlled by tandem promoters in E. coli. In that study, it was measured and then modeled two main forms of interference. One is when an RNAP is on the downstream promoter, blocking the movement of RNAPs elongating from the upstream promoter. The other is when the two promoters are so close that when an RNAP sits on one of the promoters, it blocks any other RNAP from reaching the other promoter. These events are possible because the RNAP occupies several nucleotides when bound to the DNA, including in transcription start sites.
Similar events occur when the promoters are in divergent and convergent formations. The possible events also depend on the distance between them.