It is only temporary, it is used to make macOS re-initialize all of its settings. Once the os acknowledges the display attached to the RX 460 card exists, you can remove it and (hopefully) never need to attach it again, unless you just want to.
Connect your monitor to your IGPU, using the HDMI connection and connect the other monitor (tv) to the RX 460 card using the HDMI connector. Make sure the monitor inputs are set to use the HDMI input connected to the computer. Avoid using the VGA connection.
Turn both monitors on, then turn on and boot the machine. Let it boot completely to the log-in screen. It can take a couple of reboots to get macOS to fully propogate the knowledge of the monitor through its system, but hopefully it will take only one try. Check the System report each time to keep track of any progress. At some point, both monitors should have the same background that Sierra uses for the login screen background, but one will have the login screen as well.
