Hi!
for multiple MyDMX interfaces, you just need to plug them all into the computer's USB ports (you can use a USB hub, but try to a higher quality one) then run the program and if the licenses are all good then mydmx 3 software will detect them all and you can assign each port to whatever universe number.
Artnet is a bit different, it comes out of the computers Ethernet port over a cat5 cable and the other end of the cat 5 should be plugged into the Node you purchase, and from there you need to change your computers IPV4 IP address to one close to the IP address of the node. it's a bit more involved for setting up and really if you are only planning on using 2x DMX universes total then it might not be totally worth it. And it is 99 dollar PER artnet universe.