There is one (1) way that it is legal. If the goods from the second pick up are of Canadian origin, and have not been advanced in any way, shape, or form, it is legal for a Canadian carrier to pick up in the U.S. and deliver in the U.S.
Example; Customer XYZ builds Pink Widgets in Montreal. XYZ stores some of their stock in Montreal, and some of their stock in Champlain, NY. A Canadian carrier can legally pick up part of the load in Montreal, and the rest of the load at the warehouse facility in Champlain, NY, and deliver it to Atlanta, GA.
Caveat: Never tell a CBP officer when you are crossing the border to do such a move that is actually what you are doing. While it is technically legal, most CBP officers will deny you the opportunity, regardless of the law/ruling.
If that is something your customer wants you to do regularly, have the customer clear it with the port supervisor first, and make sure they get a letter stating CBP's the supervisor's position on the matter.