You would need two ports able to accommodate enough traffic that it would effectively be the largest two ports in the Americas. You are also going to really mess up logistics as you are splitting one transit trip to three different companies. Costs also rise when you have to process cargo, which you are now add two additional times process cargo.

It isn't going to be economicly viable. Hell, China's plan to cross Central America also includes a canal due to cost.

the railroad part exists, and predates the canal even!