If you grow different varieties of azaleas, there's nothing you can do to get them to bloom at once. Each variety has its own individual timetable. If they're all the same variety, they should bloom pretty much together if given identical growing conditions. For example, plants that receive more moisture may bloom earlier or later than plants of the same variety that receive less moisture. Thus, to synchronize their flowering cycles, give like plants the same amount of moisture, fertilizer, and exposure to sun and wind. Also, test the soil to make sure that different planting areas contain similar amounts of organic matter and have the same level of acidity (pH).