So if I connect the two converters for a MacBook (with a USB-c cable between) to two power outlets. What will happen? See the picture.
Nothing will happen.
You are not directly connecting anything from the mains side of the power adapter to the USB side of things. The low voltage side (USB) actually isolated from the high voltage side. The highest amount of voltage being passed is +/- 5V which is more of a data sense connection. It’s looking to see if there is a device making a request to be charged. Since power adapters don’t make this request by design, it will never send higher voltage.
Now, it's not fair to say absolutely nothing will happen as it's conceivable that you might blow the USB controller (the chip that does the negotiation). However I would bet that the engineers designed the circuitry to be one way only. I don’t have access to the schematics, but I would bet there are diodes on the 5V connection’s to ensure a one way connection - they are there on the older MagSafe adapters so it only makes sense they would still be there on the newer USB adapters (see above link)
From a pragmatic point of view, engineers would design to accommodate this sort of behavior - they design things to fail safe to the user not fail to protect itself. That said, don’t do it because the shock you'll get is the one that comes from having to replace two previously perfectly good power adapters.
This is a bad idea and you should not do it. This creates a physical connection between sockets which opens the potential for damage or danger.
That being said, it's likely that nothing will happen. Both USB-C chargers here follow the USB Power Delivery standard, and will only send or receive power after communicating with the device on the other end of the cable. They should detect the dangerous configuration and do nothing. But you're depending on these two adapters working ideally so that you avoid starting a fire. It's just not worth it.