What do you gain?
Headaches and little performance gain in games.
Unlike older games, modern or even semi-modern games are not designed with SLI or Crossfire setups in mind. Usually, you will see no performance benefit over 1 2080ti in modern games. A lot of modern games will also have headache-inducing issues on SLI or crossfire setups such as stutter or visual artifacts.
Some other applications can benefit from SLI, but I would imagine your performance in photo editing is already enough that you would see little gain over 1 GPU from another 2080ti, assuming your application supports multi gpu scaling.