reftex-index-phrases-logical-and-regexp

reftex-index-phrases-logical-and-regexp is a variable defined in `reftex-vars.el'.
Its value is

" *&& *"

Documentation:
Regexp matching the `and' operator for index arguments in phrases file.
When several index arguments in a phrase line are separated by this
operator, each part will generate an index macro. So each match of
the search phrase will produce *several* different index entries.

Note: make sure this does no match things which are not separators.
This logical `and' has higher priority than the logical `or' specified in
`reftex-index-phrases-logical-or-regexp'.

You can customize this variable.