There can be only one spot for the development of Samadhi. If there are many, there will be no development. We can put the point of concentration in various places such as the tip of our nose, the breath, the forehead, within our body or mind. It can be anything which is considered to be the starting point.
The starting point is called raising thought conception (Vitaka Citta), that is, to lift the mind up to the point of concentration and to fix the attention there. After that, discriminate and contemplate that object of concentration until all is clear and comprehensive, until there is an understanding of matter and mind (consciousness plus the various mental factors which condition consciousness). Joy will follow automatically.
If the mind is not fixed on the object, Samadhi will not be there. If the mind moves from one point to others, again Samadhi will not be there, it is simply ordinary thinking. So it is said that our body is the principle or base of mindfulness and the base of Samadhi.