I don't know of anything that will make them bloom reliably a second time same year. Occasionally some do, other years they do not and it seems to be weather dependent. The second round of flowers in Fall is also thought to be characteristic of particular plants and not necessarily all plants of that cultivar.

This is not normal and when it does occur it usually results in mediocre flowers from otherwise good buds that would bloom beautifully in the spring. It occurs when the weather has weird temperature cycles.

The one exception is Encore Azaleas. (Azaleas are a type of rhododendron) They are bred to do this and will do it fairly well in the south, not very well in the north.

Here is a link that might be useful: Encore Azaleas