You should not observe voluntary fasts when you still owe one or more days from Ramadaan, rather you should start with the fasts that you owe from Ramadaan, and then observe voluntary fasts.
If you fast the tenth and eleventh of Muharram with the intention of making up days that you owe from Ramadaan, that is permissible and will make up for two of the days that you owe. The Prophet (peace and blessings of Allaah be upon him) said, Actions are judged but by intentions, and every person will have but that which he intended.
Fataawa al-Lajnah al-Daa'imah, 11/401
We hope that you will have the reward for making up the missed fast and the reward for fasting that day.
Fataawa Manaar al-Islam, by Shaykh Muhammad ibn Uthaymeen (may Allaah have mercy on him), 2/358.
Reproduced from Islam QA