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.
WHEN it actually works, the gains are SUBJECTIVE TO THE USER'S PERSONAL INVESTMENT in such a setup.
It can also pigeonhole you into playing only the well-supported titles, because when it doesn't work... you now have a $1000+ card sitting there doing a whole lot of nothing(not taking into account the other little 'intricacies')
If I had blown $2000+ on 2x 2080Tis, I could see myself only playing games that SLI worked well on...
So I could further enforce the self-denial that I wasted $1000+.