Madhusudan’s body is a vessel and Sathya Sai Baba’s ‘Pragya‘ is the water – the content.
Madhusudan emptied himself of his own personality (Aham Bhava) to make space for Sathya Sai Baba to fill his being with His Pragya. Therefore, this form which has the body of Madhusudan but Pragya of Sathya Sai is – Madhusudan Sai.
Just as the Pragya of Shirdi Sai manifested in the body of Sathyanarayan Raju and was known as Sathya Sai, so is the Pragya of Sathya Sai manifesting in Madhusudan and is now Madhusudan Sai.
So, the outer form is Madhusudan and inner content is Sathya Sai.
However, the way you address Him that is the way He will respond. Call Him as man and He will respond as man. Call out to Him as God, He will respond as God. However, when not addressed either as human or divine – he is the combination of both divine in human and human in divine – as Madhusudan Sai.
Earlier, the choice of responding as human or divine was with the divine but now the choice is with human. Like the control button that switches the channel from one to another, which was earlier with Sathya Sai alone, now it is with Madhusudan Sai.