Wait what? Normally different tier GPUs CAN'T WORK IN SLI, crossfire maybe. And 1070ti is not a rebranded 1070 it's more like a 1080ti than a 1070. So no, it's probably not going to work in SLI.
It's not worth it at all. You're more likely to sell both cards or refund the new one and just get a 1080 or 1080ti even. 1080ti will have kinda the same performance as a SLI 1070.