The Assam and Meghalaya data is combined because that is the way it is represented in the date set. You would need to have the data recollected or reorganized at the data source level to get the correct measurements for those to become two separate data points.
There is some excellent information on customized filled maps that can be found here: City Map for India
As well as here: Create Your Own Filled Maps in Tableau - Clearly and Simply
But again they are going to require some data manipulation to have the Assam and Meghalaya regions/states populated correctly.