Route stereo audio from channel 1->3 & 2->4 using 4-channel sound interface

How can I mirror the stereo audio output on output channels 1 & 2 to channels 3 & 4 on my 4-channel sound interface (Focusrite Scarlett 2i4) using free software without adding more cables? Currently I am successfully mapping channel 1 to 3 and channel 2 to 4 with the trial version of Rogue Amoeba Loopback, but the trial version only lasts 20 minutes. I have tried to do this with "Audio MIDI Setup" but it is not supported. Bear in mind this is on a 2012 Macbook Air locked to macOS 10.9, so it cannot be upgraded (ever).