I did not see this anywhere, however I think what I am going to say is true.
The Gemara in Megila 7a-7b (translation courtesy of ArtScroll) says as follows:
R' Yehuda Nesiah sent to R' Oshaya the thigh of a calf born third to its mother and a bottle of wine.… Rabbah sent with Abaye to Mari bar Mar a basket filled with dates and a cup filled with the flour of roasted wheat.… Mari bar Mar sent back to Rabbah a basket filled with ginger and a cup filled with long peppers.
We see from the first two stories that they sent two different Brachos.
Although from the final story we see that both items were the same Bracha, I would venture to say that, since the first 2 stories of Mishloach Manos were of 2 different Brachos, that is where the misconception started that you need the 2 items to be 2 different Brachos.