Apart from the general document layout, the layout and formatting of indexes depend on two things:
The parameters of
.ist(index style) files that are used by
makeindexwhen transforming raw index data into
.indfiles (which contain a
theindexenvironment). These parameters control, e.g., the presence of letter headings to each index group.
It is possible (although often not sensible from a typographic point of view) to change the definition of the
theindex environment mid-document (e.g., "typeset[ting] a general
index in two columns and then switch[ing] to three columns for an index of names",
idxlayout manual, p. 4). But is it possible to change
.ist (index style) file parameters mid-document (e.g., adding letter headings for groups in some, but not in all indexes)?
EDIT: To clarify: My aim is not to switch
.ist parameters within a single
theindex environment, but to use different parameters (or achieve a different formatting) for several indexes within the same document.
EDIT 2: I take Martin Schröder's answer as a hint that it is possible to run
makeindex "by hand" several times and to choose different
.ist files each time. On the other hand, Leo Liu states in a comment that one "can add TeX boolean switches in the
.ist format and modify
\index to make it work", i.e., control the formatting through LaTeX macros. I'd be interested in a concrete example of a boolean switch within an