Efflock prevents efflorescence and creates hydrophobic tile bed, adhesive and grout for around $3.87/m2 (for a 30mm thick tile bed). Compare that with the cost of a secondary membrane for at least $20/m2, and you're way in front Even with a tile bed 100mm thick you're saving cost and extra processes.
Plus there are no hassles with inclement weather, no issues with trapping moisture between two membranes, and you even have the option to wet bed your tiles! Efflock allows the bed to permeate, meaning it will release moisture from the hydration process, whilst repelling water ingress.
Less cost, big time savings and best of all, no heart-breaking efflorescence stains!