A Shalach Manos Gift from the Rashash

The Rashash has comments on the Gaon's Ma'aseh Rav, and in #249 he says that he believes that when Reuven sends Shalach Manos to Shimon, and Shimon accepts it, then they both were mekayeim the Mitzva of Shalach Manos.  This is why the mitzva is referred to as משלוח מנות, in a noun form, instead of שולחים מנות, in a verb form.