They are both flavouring chemicals (quick search on Google suggests they give nutty and chocolate like flavour/odour), but aren't diacetyl. The two chemicals combined don't equal diacetyl. Another name for diacetyl is butanedione and there are
some others.
Di just means "two" of something. A famous prank was a campaign to ban dihydrogen monoxide, which listed the dangers of this chemical (fatal to inhale etc.) and describing how it was the principal ingredient of acid rain, and used to cool nuclear reactors. Dihydrogen monoxide is of course another way of describing H
2O or water