I think your psu will not handle 2x gtx 1080ti, but i think it can handle 2x gtx1070ti.
And my guess this gpu will be your best bet. imho.
Thank you. That's a helpful start.
Would it make sense to upgrade my PSU to be able to support 2 1080ti?
Which manufacturer would you recommend? There are many different 1070ti. I am partial to the 3 fan look, but really am not that knowledgeable about parts overall.