Using two different graphics card on one machine

I have a machine with a GTX Titan X (GM200) installed, and I’m planning to add another card such as GTX 1080 Ti or Titan X Pascal. This machine runs Ubuntu 14.04 LTS. So is it necessary to install two different drivers for those two cards, or the Linux driver for GTX Titan X works for both cards?

You cannot install two different drivers. A single driver can support both. However you may need to choose a recent enough driver so that the driver supports the types of GPUs you want to use. I’m not suggesting that whatever driver you have installed now will support any GPU. You may need to install a newer one.