Praise be to Allah.
You should not observe voluntary fasts when you still owe one or more days from Ramadan, rather you should start with the fasts that you owe from Ramadan, 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 Ramadan, that is permissible and will make up for two of the days that you owe. The Prophet (peace and blessings of Allah be upon him) said, “Actions are judged but by intentions, and every person will have but that which he intended.” Fatawa 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.
Fatawa Manaar al-Islam, by Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him), 2/358.
And Allah knows best.