How many PCI-E lanes does each port get?
Do the ports share the channels/lanes?
Do they both support Power Delivery? How many Watts does the thinkpad need to charge?
Even with 'clear' labelling, it can still be a crapshot as to the full capabilities of the device.
Right now I have an Uptab Mini-DisplayPort adapter in one port, and a StarTech full size DisplayPort adapter in the other. The Uptab adapter has its own USB-C input for power, so that is where the power supply is connected.
The mini and full size DisplayPorts are each connected to a 4K monitor, and I use all three - the two externals and the ThinkPad's WQHD. This all works great when the Uptab is in the rearmost USB-C port and the StarTech is in the other port. But if I switch the two around, one of the monitors goes into low resolution mode!
This kludgy configuration is just because these were the adapters I had on hand. There are a number of adapters that plug into a USB-C port and give you two mini-DisplayPort outputs - I have been meaning to try one out.
And therein lies the problem.
Used to be, we could call a USB port a USB port. Everybody knew that if the computer had the port, it could work with the peripheral; if the computer didn't have the port, it would not work with the peripheral. If the peripheral required different functionality then it required a different port that was equally straightforward and obvious.
Now, it's all supposed to run over USB - at least, over some USB. Consumers go out and buy a device and then are confused when it doesn't work. They know their computer has a USB port, but that's not enough. The port no longer represents a definite protocol, or even a definite power supply capacity. Are they supposed to check to see whether each of their USB ports says "USB 3.1 Type-C Gen 2 with Thunderbolt 3 (including DisplayPort)" next to it (which it doesn't)?
In fact, my ThinkPad X1 Yoga has two USB-C ports with Thunderbolt icons next to them. They support USB 3.1 Type-C Gen 2 with Thunderbolt 3 (including DisplayPort).