Praise be to Allah.
Yes, there is nothing wrong fasting the six days of Shawwal on Mondays and Thursdays, and there will be recorded for you the reward of fasting the six days (of Shawwal) and of fasting on Mondays and Thursdays .
Shaykh Muhammad ibn ‘Uthaymin (may Allah have mercy on him) said:
“If it so happens that the fasting of these six days of Shawwal is done on Mondays and Thursdays , then the person will attain the reward of fasting on a Monday with the intention of attaining the reward for fasting the six days , and the intention of attaining the reward for fasting on a Monday or Thursday, because the Prophet (blessings and peace of Allah be upon him) said: “Actions are but by intentions, and each person will have but that which he intended.”
And Allah knows best.