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.