The |et filter [1] (since SPIP 2.0) is used to check the existence of both of two elements.
Wikipedia offers a complete definition of the Logical conjunction.
Examples:
#
[(#CHAPO|et{#PS}) There is a stand-first and a post-scriptum ]#
[(#CHAPO|et{#PS}|non) There is not both a stand-first and a post-scriptum (but there may be one or the other) ]