If you're using an AMD GPU with the closed-source AMD driver ati-drivers then install the Entropy package amdcccle (AMD Catalyst Control Center Linux Edition) and use that (it has a GUI). If you're using a NVIDIA GPU with the closed-source NVIDIA driver nvidia-drivers then install the Entropy package nvidia-settings (NVIDIA Linux X11 Settings Utility) and use that (it has a GUI).
If you're using an open-source driver then try using xrandr. See, e.g., How To Setup Dual Monitors With XRandR
and X.Org/Dual Monitors