Multiple UNOs R3 and R4

When your projects are growing you might come to a point where you decide to use more than one Arduino connected to your PC via USB at the same time. How does the operating system handle this? In the times of good old MS-DOS, only COM1 to COM4 were possible, but thanks to USB, even three-digit numbers are no problem:

(Sorry, Microsoft translated all the entries to the user's language. Hopefully, you will be able to understand their meanings.)

The IDE even offers selection of non-existent ports. Let us start using two ARDUINO UNO R3:

Before uploading, make sure you have selected the desired port!

The IDE reports WRITING and also READING in case you have activated VERIFY in the PREFERENCES before UPLOADING.

Now let us connect an ARDUINO UNO R4. The sad news is I got this error message (still using Windows-7)

Nevertheless, compiling and uploading to the R4 was working. To confirm uploading really has been performed, I added various numbers of short blinks to the standard blink.ino example.

Now it is time to connect a second UNO R4. Let us have a look at the device manager:

(Some lines of the device manager window had to be deleted in order to avoid reducing the size of the picture.)

There are no more COM-ports replacing a USB device but two CDC ports with equal names. That should lead to problems. And it does.

Now the port selection offers one group of Serial ports and another group of dfu ports. No matter which one you select, trying to upload will give you this error message:

More than one DFUcapable USB device found! Try '--list' and specify the serial number or disconnect all but one device

So you may only connect one UNO R4 at the same time when you are going to upload your sketch.

Now the IDE surprises with DOWNLOAD and UPLOAD simultanously. Where exactly is up and down? I am getting confused.