Dual 4k monitor setup at 60hz with 2015(?) 15" MacBook Pro
You should be able to run two 4K monitors at 60hz. However, you'll probably be stuck at 30hz if you are using HDMi. If it is in fact a 2015 MBP, there should be a few thunderbolt 2 ports. I'd suggest using a mini display port -> display port cable that supports Display Port 1.2 and thunderbolt 2. Something like this should do the job. Also, since I don't know the exact model you have, I'd hold off on buying the second monitor until you know what your MBP is fully capable of.