What is the meaning of 'successive dynasties'?
how could one family have two dynasty?
You're misinterpreting the passage.
The Kushans were a culture/society that formed an empire in India. It says that there were two families that formed two separate ruling dynasties. The first was the Kadphises and the second was the Kanishka. The Kanishka directly followed on from the Kadphises, therefore they are successive dynasties.