Do I need two computers?

Yes. The card goes into the PC you play on, and a second machine reads it and runs the software. That separation is the whole idea, so one computer cannot do both.

The second one does not need to be powerful. A cheap used mini PC, an old office desktop or a laptop you stopped using is fine, as long as it has 4 cores or more, 8GB of memory and a USB 3 port. For MiTM it can't be a laptop. Your Second Computer lists good cheap ones.