First, check whether your discrete graphics card has two (or more) connectors:

Second, connect two monitors to the graphics card. As shown in the figure:

Thirdly, a main display and an auxiliary display are arranged.

Take win 10 as an example:

1. Right-click the desktop and select Display Settings.

2. Select the monitor numbers that need to be set as the main monitor, then check "Set as the main monitor" and select "Expand these monitors".