This is what you can try to do, it might work.
Once the second slingbox is installed > disable the UPNP on your router and assign another port to the MAC address /or static IP of the second slingbox.
Reset the 2nd slingbox and run once again the setup and this should help you add a new slingbox in your network
I understand that you would like to add a second Slingbox M1 to a different cable box and you have your first Slingbox M1 assigned to port 5301. Below is a helpful link:
"If you have more than one Slingbox on your network, each one receives its own port number, incremented by one: for example, port 5202, port 5203, and so on ... up to 5225."
Sling Media Support!