You can create as many direct transfer mass flux links as desired in order to build your pathway network. There is, however, one limitation that you must keep in mind regarding the use of direct transfer mass flux links: Direct transfer mass flux links are uni-directional. That is, mass can only flow in one direction (negative transfer rates are not allowed). If you wish to simulate a reversal of the transfer direction, you must create two separate direct transfer flux links to represent this (one flowing in one direction, and the other flowing in the other, with the transfer rates defined using expressions which ensure that they are never negative).