Hi Guys,
Guest OSes in Parallels Desktop have no access to physical graphics cards present in a Mac. Instead, Parallels Display Adapter driver (which is part of Parallels Tools installation) interfaces with virtual hardware and provides 3D acceleration features. The actual acceleration is achieved by translating DirectX commands from the guest to OpenGL API on OS X side. Since you are using discrete graphic cards, please set "Better Performance" To switch between the adapters, go to virtual machine configuration window -> Options -> Optimization - Power.