They mean approximately the same thing, but 掩飾 means to "cover up" whereas 隱瞞means "to hide." In Chinese, as in English, you're more likely to "cover up" a MISTAKE and "hide" a BAD DEED or SECRET. But both mean to prevent people from knowing something you don't want them to know.

隐瞒的特点在【隐 hide】 掩饰的特点在【饰 cover】

隐瞒 just means that you don't tell the truth while 掩饰 can mean you do something else to cover up the truth you are not willing to tell others/ your real feelings/ your mistakes. For example, 我应该告诉他真相的，可是那时我隐瞒了下来。 他把话题转移到天气上，试图掩饰他的窘迫。