Either you buy a monitor that can charge the laptop with USB Power Delivery, or you would need a DisplayPort adapter that provides an extra port for power passthrough.
If you want everything in one cable, IIRC all announced Thunderbolt 3 docking station can drive two 4K displays with varying combinations of ports.
You'll need a TB3 dock to do that effectively. The Caldigit TS3 Lite is the only game in town, all the other ones aren't shipping yet. You get 1x DisplayPort and 1x USB-c which can be converted to DisplayPort for 2x 4k, and the other port on your MacBook plugs into the charger.
I would go with the Dell Ultrasharps since thats what I'm used to, but being able to change the brightness from the keyboard controls is huge on the LG. Its the little things.