No, you cannot (at least for most operation cannot). For compute (e.g. OpenCL), some software can utilise both GPU, but I don't think that's why you want 2 different GPU in a single a machine. For something like 3D modelling, cannot, only the GPU that connected to the monitor is working. Of course you can connect both GPU to a single monitor, and switch the monitor's input source to utilise different GPU, but I don't think that's what you mean "use both GPU together".