Connecting to tcp client programs (sockets) together using netcat

I have some device which is on the network layer acting as TCP client. And I have some programm on the remote host (debian machine) which is acting simililary as a TCP client on the network layer. How can I forward data back and forth between those two pieces of software using netcat. I know that i can establish a connection to the device by means of netcat this way "nc -l -k -p 4555". But I don't now how to forward data to the client programm after.