Why 2 equal decoupling capacitors are needed for each STM32 VDD pin?
It is not recommened to blindly believe what is said in random videos on the Internet. There is no such requirement or recommendation for STM32 MCUs bypass caps if you do the fact checking. It might just be a personal opinion or rule of thumb for the person who made the video and it is just mentioned without a disclaimer to confuse others with false facts, or just genuinely wrong without knowing it.
So, two 100nF capacitors are not needed, only a single 100nF capacitor per supply and ground pair is needed.
And even if two 100nF was the recommendation, two 100nF capacitors is not equal to single 200nF capacitor. The capacitance will be equal, but bypassing is not only about capacitance, but impedances. Two caps in parallel have half the ESR and ESL than a single capacitor, so if these parameters are needed then it is common to parallel two or more smaller capacitors to end up with a better capacitor than a single larger one.