Small question about concerta. Can I give two tablets for the total dose?

My daughter was taking concerta 18 mg. The doctor increased the dose to 27 mg for 7 days and if we didn't have good results with the 27 mg we had to increase it to 36 mg afterwards. We have to start the 36 mg tomorrow morning but I forgot to pick up the tablets today for tomorrow morning. So my question is, can I give him two tablets of 18 mg (to make 36 mg) for tomorrow morning? Thank you