Sections 327D.1 through 327D.29 shall not be construed to prohibit the making of rates by two or more railway companies for the transportation of property over two or more of their respective lines within the state; and a lower charge by each of said companies for its portion of such joint shipment than it charges for a shipment for the same distance wholly over its own line within the state shall not be considered a violation of this chapter, and shall not render such company subject to any of the penalties of this chapter.