How to use two internet connections independently
This is an admittedly convoluted method, but you could run the torrent app in a vm. Vbox, Parallels, Fusion, whichever. Then you can bind the vm's networking to a specific network interface. The default for the vm networking is usually "Auto" which means it uses what the Mac uses and that's dictated by the service order in the Networking System Prefs. But you can also bind to a specific interface and that will keep the vm's traffic on that connection.