There are many different options out there for mixing your Boxcast video. The below options cover only a small fraction of them. Feel free to make a choice that suits your broadcast needs.
While we will certainly help you if you run into issues when implementing a mixer but your best bet will always be to contact the actual product manufacturer themselves as they will have a far better idea of the limitations and uses of their product.
Note: You will need a video capture card to bring your camera signal into the computer. One capture card per camera. The Black Magic Intensity is a good option.
This is by no means a complete list but it should help you get started comparing your options.