It will be decided by the law of the state, police which law will be considered first, the opinion of the court will also be important. Both states involved since the incident took place in Mumbai and Sushant was from Bihar, the two states related to it. In general, when we report to the police about a major crime. Or make a report to the police. So it is called FIR. The FIR runs according to CRPC 1973. Any person can report any incident to the police. And if he cannot go to the police during the incident. So he can register his complaint over phone also. But if you complain on the phone. Then you have to go to the police and write FIR to them later. You have to register two complaints on the phone to the police, later you will have to go to the police station and register that complaint.
In general, the FIR is processed under Section 154 of the Indian Penal Procedure Code 1973. In Sushant Rajput's case, it has been told by communication mediums that Mumbai Police has filed Accidental Death Report in Sushant case. He has done this under Section 174 of CRPC. In such a case, the FIR filed in Bihar will be considered as the first in this case.