BrickUnderground

The proprietary lease for my coop defines a lessee as someone who "is a daily occupant of the building for a minimum of nine months of the year" and says "The Lessee shall not occupy or use his/her apartment or apartments for any other purpose than as the personal and primary dwelling for the Lessee" (although if owning more than one apartment, a lessee is entitled to sublet one of the apartments). It also states "that subletting by Lessees who are not occupants of the building a minimum of nine months per year will be limited to two years in any four years period."  We have a couple in our building who own two apartments.  They have moved out of the building and have been living in another state for about four or five years.  They sublet one of their apartments and keep the other one vacant, claiming it is still their primary residence.  They show up at the building maybe once a month to collect mail and to make a show that they are still in residence. They seem to be in violation of the terms of residency, but no one knows how to definitively prove this.  Any suggestions?