Routing traffic to a specific nic

I’m replicating 3GB of data between 2 servers and have installed a private network between these two servers so that I can pull this traffic off of our production network. The copy was taking several hours and I needed a quicker solution. So I have installed a seperate nic in each server and use a crossover cable between them. I’m using a batch file in which I map a drive and then robocopy its contents. I want to be able to specify that this traffic only use the new private network. How can I do this?