I would acclimate no more than 2hrs, anything more is overkill and better left to mobile invertebrates. Acclimation is usually a stressful adventure for corals and intense lighting can be an added stress factor. It isn't going to hurt your other coral by turning off the lighting or at least 1 halide for the day. You can even run them intermittently throughout the day at short intervals for light acclimation and see how the new coral reacts. You are going to have to play around and experiment to see what works.