This is a hardware limitation.
First off, your laptop doesn't actually have two complete graphics cards. It has two GPUs, but only one is completely wired and all outputs are connected to it. This is true for most laptops with hybrid NVIDIA graphics.
Your laptop's main graphics card is Intel HD Graphics 4000, which is integrated in the CPU. It's always enabled and it's the only device in your laptop that can produce video signal on outputs. When a graphics-intensive application is running, NVIDIA GPU kicks in and starts rendering graphics. Then pre-rendered frames are sent to Intel GPU and it forwards them to outputs. For Intel GPU these are just still images and it's quite good at rendering them, so there's no performance loss.
(By the way, there are also some cases when NVIDIA GPU is used, but frames are rendered by Intel GPU - google "CUDA" or "GPGPU".)
The good news is that HD Graphics 4000 can support up to 3 screens (including the laptop one) under some conditions. The bad news is, these aren't met in your laptop, because internal screen is connected via LVDS cable and eDP connection is required for 3 monitors to work. (source) I know that it's LVDS and not eDP, because you can buy a replacement LVDS cable for this model.
You can buy an external video adapter connected through USB, but this solution is far from perfect. USB adapters have heavy impact on CPU usage and limited throughput, so you may have problems with high resolutions and framerates. I've heard that USB 3.0 adapters work with 1920x1080 screens at reasonable framerates, but I haven't tried it myself.