Can two opposite localizers to a single runway be on simultaneously?
ICAO Annex 10 Volume 1 has the answer:


  3.1.2.7 At those locations where two separate ILS facilities serve opposite ends of a single runway, an interlock shall ensure that only the localizer serving the approach direction in use shall radiate, except where the localizer in operational use is Facility Performance Category I — ILS and no operationally harmful interference results.


In plain language: only one localizer can be on at a time, unless the ILS in use is only CAT I and it has been confirmed that no harmful interference will happen.