Sonne has an OLD Maxwell Titan X, not a Pascal one.
The games you play do support SLI? Because these days that is bit terrible.
As you will see on the video Feoilban posted above they test many games at 4K and you can compare it to the performance you get with your two TXs.
IMHO, sell the TXs, buy a single GTX1080Ti, watercool it and overclock it with the left over money.
You won't have to worry about SLI support and can perform much better than aircooled.
Alternative, wait for the Vega to see what it has up it's sleeves.