ANSWER:

What he has said is wrong. No problem in having double intention except for fasting the 6 days of shawwal as long as one is mandatory and the other is voluntary. 

http://localhost/assimnet2018/11/24/if-i-choose-to-fast-the-sunnah-of-monday-thursday-can-this-count-as-making-up-the-missed-fasts-during-ramadan-or-do-i-have-to-have-separate-intentions-for-one-fast/

Sheikh i heard from the scholar in india saying that we cannot make two intentions while fasting.. such as fasting the nafl fast ( mondays and Thursdays, 10 days of dhul hijja , 6 days of shawaal etc) to recover my farz fasting ( ramzan). Please can u explain can a recover my farz fasting while fasting on the nafls fasting days.