Puma, as Randy and Skuzzy already said, you won't gain much using SLI (was thinking about LSD and figured it's not the right abbreviation here).
My humble advice is to build your new rig around that 1070 and use it as long as it serves you at the level you want. Having a spare will end up into having an expensive piece of outdated hardware gathering dust. When the 1070 is out of warranty and gets issues, you'll want an 1170 or later instead of a similar one.