This could result in some difference in concentration of the insulin. Know the total amount of the two or more insulins so that you will know the right point to stop on the syringe. Many people make mistakes in adding, so you should double check this total. Blood sugars should be watched carefully when the insulin bottle is almost empty. If the blood sugars start to be unusually high or low, the last bit of insulin should be discarded.
Some people prefer to just routinely discard the last bit of insulin in a bottle.
The expiration date on the bottle should always be checked and the insulin discarded if that date is reached. An overdose can cause an insulin reaction or low blood sugar. If you ever take an incorrect dose, be sure to notify your diabetes care provider. It is wise to have the morning and afternoon dosages posted on the refrigerator or some obvious place to prevent confusion. Store cotton balls and rubbing alcohol in a container with a lid.