Connect two USB-C displays to MacBook Pro 15" 2018
You should absolutely be able to do this. From Apple's web site


  Video Support
  
  Simultaneously supports full native resolution on the built-in display at millions of colors and:
  
  Up to two displays with 5120-by-2880 resolution at 60Hz at over a billion colors
     Up to four displays with 4096-by-2304 resolution at 60Hz at over a billion colors
  
  Thunderbolt 3 digital video output
  
  Native DisplayPort output over USB‑C
     VGA, HDMI, DVI, and Thunderbolt 2 output supported using adapters (sold separately)


However USB-C/Thunderbolt can be a bit finicky/picky about cables and monitors. Perhaps if you shared your exact configuration (cables/lengths, monitors, etc.) we might be able to help. Just add that info to your original question.