Apparently You can prune most Magnolias twice a year if you want. The traditional time is after the spring flowering, but if you do half then and half in late Autumn you may be able to get them into a cycle of flowering twice a year.
I've done this to the grandiflora in the garden but it's in a down cycle at the moment because I had to prune very heavily all round in the spring because of lack of space, I'll be pruning next w.e. if the weather stays cold, to try and start the cycle again.
