reftex-index-phrases-logical-or-regexp

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

" *|| *"

Documentation:
Regexp matching the `or' operator for index arguments in phrases file.
When several index arguments in a phrase line are separated by this
operator, the user will be asked to select one of them at each match
of the search phrase. The first index arg will be the default - a
number key 1-9 must be pressed to switch to another.

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

You can customize this variable.