Are you ok with a plain C, comand line application. I would recommend a solution in C. It is much faster and uses less CPU. It can also be done in C# if you really need that.
App should listen to all traffic on a specific port on NIC 1. That is OK. Broadcasting on NIC 2 i dont really get. What will be on NIC 2, only your local network? So, the packets will be send to all the computes in your local network. One must specify IP address of each individual PC where you need to broadcast, so it only makes sense if you need to broadcast to the local network. Then there is the question of port where to broadcast. Probably the same port as on the NIC 1 side. Question of delivery? Do you need a guarantee of delivery? Will there be traffic going back from NIC 2 to NIC 1, any sort of reply?