Answered by Shaykh Faraz Rabbani
Are we not supposed to fast on the 11th of muharram? And is it necessary that if we voluntarily fast (that is not in ramazan) are we supposed to keep 2 fasts together and not 1 fast alone?

Walaikum assalam wa rahmatullah,
I pray that this finds you well, and in the best of health and spirits. May Allah grant you all good and success in this life and the next.
There is nothing wrong with fasting the 11th of Muharram, or to perform one voluntary fast alone. In situations where fasting the 10th of Muharram would have entailed imitation of non-believers, it would be better to fast it with a day before or after.
[ Ibn Abidin, Radd al-Muhtar ]
And Allah alone gives success.
Wassalam,
Faraz Rabbani