You would not need an ejection charge from the first stage to separate the upper stage----as soon as the upr stg motor fires up, the stages will come apart. This is the normal method for staging BP-to-BP. The booster motor (lower stage) does not have an ejection charge. The last bit of propellant in the booster motor burns down to a thin wall and then bursts forward, pushed by the pressure of the booster combustion chamber. The thin wall fractures into bits which (usually) go into the nozzle of the upper stage motor and ignite it. All this takes place while the two stages are still hooked together. After the upr stg fires up, the motor exhaust pressurizes the inter-stage volume and pops the booster loose.