iMac Pro running 2 LG Ultrafine 5k, blurry
Most likely, you are plugging the displays into the same Thunderbolt controller (ports adjacent to each other).  To ensure you're using the other controller, use the ports on the "opposite side" of the bus

According to Apple:


  Note: The Thunderbolt 3 ports on your iMac Pro are managed by two
  controllers, one for the two left ports, the other for the two right
  ports. Each controller can support two 4K displays or one 5K display.
  So you can connect up to four 4K displays or two 5K displays to your
  iMac Pro. To connect two 5K external displays, for example, connect
  the first 5K display to either of the first two Thunderbolt 3 ports,
  and the second 5K display to either of the last two Thunderbolt 3
  ports.