Hi! I think (I may be wrong) that it is just two ways of saying the same:
Think that we have a vector XY that is a concat of two other vectors (X and Y). Lets say that X has n x 1 as shape and Y mx1. Then, XY is (n+m) x 1. Lets also say that X has its weight matrix and Y too, and both matrixes map to the same dimension k. So Mx (X matrix) has k x n as shape and My has k x m. You can also say that there is a Mxy matrix that is a (concat?) of Mx and My (with shape k x (n+m)) and
Mx X + My Y = Mxy XY
If I just said bull* im sorry