See this thread for a similar question (for two Shapeokos, but same difference)
I never tested that configuration (since I only have one C3D machine…), but I have a feeling that since Carbide Motion does not allow to select a specific COM port, and picks the one it finds that matches the C3D vendor id/product id, a second instance of CM would (probably?) not find the second Nomad.
Now, if this is important for you, you could consider using an alternate G-code sender, they usually allow to select a COM port of your choosing, and then it should be possible.
OR someone with two C3D machines can try and launch two instances of CM and confirm what happens.