If you use a multichannel sound card you can map channels to different outputs in Audacity which would achieve the desired effect and be free. I did it last year to output tracks to different amps at the same time. here is a link showing how: https://www.youtube.com/watch?v=zu37UaVlLJE
A thought for the audio amp setup. I am using small footprint amplifier boards and adding a power supply. I then mount them in an outdoor enclosure that is watertight and lockable. I get the amp boards from Parts-express.com and the power supplies and enclosures from Holidaycoro.com. You can get a four channel amp on a single board for about $40, total cost less than $100 and will offer plenty of power for an outdoor use. The amps have RCA jacks for inputs and barrier strip for outputs.