Can two opposite localizers to a single runway be on simultaneously? In case of a single runway with two separate ILS systems (one in each end, facing localizers) operating on two different frequencies, can both localizers be on and transmitting at the same time, or must one be switched off if the other is on?

Please refer to ICAO recommendations if possible.
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.