"Enhancers" of enhancers?
If you're looking for a strictly and directly DNA-DNA mediated effect (no histones, no transcription involved) I'd look for sequence effects on chromatin remodelling, modulating access of transcription factors (TFs) to their elements. Something about this might be in this paper: 
Szerlong, H. J., &amp; Hansen, J. C. (2011). Nucleosome distribution and linker DNA: connecting nuclear function to dynamic chromatin structure. Biochemistry and Cell Biology = Biochimie et Biologie Cellulaire, 89(1), 24–34. doi:10.1139/O10-139