This was bad advice. Buying two Titan X will provide nearly identical performance to two 980Ti and it will cost twice as much. The 6GB of RAM in a 980Ti is enough even for 4K.
If you aren't getting performance you are happy with on two 980 Ti in SLI you won't be happy with the performance of two Titan X in SLI either. The fact is that 4K is very tough to pull off if you want max settings but a 980Ti SLI system is definitely the way to go if that's what you want.
This chart shows an average of performance across many games at 4K. A reference 980Ti is nearly identical to a reference Titan X. The factory overclocked Strix model scored better than a Titan X at 4K. Don't waste your money on Titans.
Here is the full review in case you want to see benchmarks in specific games.
https://www.techpowerup.com/reviews/ASUS/GTX_980_Ti_STRIX_Gaming/1.html