I have a single 1070 and it does everything I ask of it.
If you're just upgrading, posting the rest of your specs will help the guys here give you better advice.
Depending on those, it might even be more beneficial to upgrade your CPU than getting a second video card.
As far as liquid cooling goes, I hear that the pre-manufactured closed systems are the most reliable for CPUs, but I have no idea about GPU liquid cooling. I would imagine the same.