Even if you would be using the second card, the 780 (non TI) should be almost 4 times weaker at this point. So I don't see how or why would anyone bother.
Unless you plan to use them as two separate cards, you don't need the old one. The 1080ti can be used on 5 monitors by itself and for rendering you'd get a better performance boost if you're using a CPU with more cores, rather than a 5 years old GPU.