chapterbib and bibunits are two LaTeX packages which
produce multiple bibliographies in a document. This is no problem for
RefTeX as long as all bibliographies use the same BibTeX database
files. If they do not, it is best to have each document part in a
separate file (as it is required for chapterbib anyway). Then
RefTeX will still scan the locally relevant databases correctly. If
you have multiple bibliographies within a single file, this may
or may not be the case.