So I purchased a second one of the USB capture devices and using Gaspars system of looking in quicktime I could see both sources however in Vision the second card could be seen but just showed black and if I selected that it caused vision to crash. Both input cards work fine on their own. I tried the same thing on my Alienware windows 10 machine and viola! works fine, two separate inputs on two separate screens.
I did have a few issues seeing the cards on my Mac laptop depending on which ports I was using, I'm wondering if an external powered USB hub would do the trick. I'll let you all know how I get on.