After developing a roll, both developing agents may be quite active, but the problem is that it depends much on the film you shot, scene content etc., so you can't exactly know how active they are, and you can't just test it as it may be slightly different every time.
The idea is that the developer yields the same result every time regardless of conditions like film and scene content, so there should be some safety margin. So, you can (ab)use this margin to process extra films. You may get just perfect results, but it may be lower in contrast due to exhaustion of phenidone or hydroquinone or both.
There is also bromide and iodide released from the film in the development process (from AgBr & AgI which became Ag) and they retard development even if developing agents would still be ok. I would guess they affect shadow tonality most.