The only issue I can see is that baking soda, especially if the box has been opened, will draw some moisture from the air. One way you can tell that baking soda has some moisture in it is that it becomes lumpy. There is probably some moisture content even in an unopened box of baking soda.
Neither baking soda nor castor oil is attractive to bacteria. Moisture, on the other hand, is attractive to bacteria. This is why, in cosmetic formulation, any product that contains water must also contain a preservative. Normally, it is suggested that any formulation that contains water should be kept refrigerated and used up within two days.
Since there is a risk of moisture contamination, I would not suggest pre-mixing. Also, I can see very little advantage in doing so.