I have two USB-C monitors (specifically Dell P2219HC), and I am trying to connect to them both as separate external displays. I am using the included USB-C to USB-C cable for each monitor.
If I plug one in, in any of the 4 Thunderbolt ports, it connects to it just fine. If I plug the second one in, in any of the 3 remaining ports, it does not detect the monitor, and the monitor displays a message indicating that there is no USB-C signal from my device. And vice-versa. I am not sure why this is happening.
I am running macOS High Sierra 10.13.6, my graphics card is the "Intel Iris Graphics 550 1536 MB".