You don’t mention what resolution you are using, and what you consider to be a bad framerate. However, i am using a similar setup, and am able to run this game at 100 ish fps at 4k ultra, with sli configured. At these settings, i am maxing out both my gpu’s and cpu (as sli lowers cpu performance). If you are running at 1440p (or god forbid 1080p), you should expect to be cpu bottlenecked, and as a result sli will scale less, or not at all.
See the link for my sli guide :