If I remember correctly, I have it set up as follows (I'm at work).
The graphics card has several connectors (hdmi, vga and DVi), the large monitor is connected via hdmi and the other monitor (old) is connected via vga to the same graphics card and I move applications from one screen to another as if it were an extension of the same.
Maybe it's not the right way but it works for me.
I hope I have helped you.
bye!