Need to enable simultaneous instances of Arduino IDE.

I quite often find myself developing with networked arduino boards and need to have two or more open at the same time - usually different types. The current IDE gets confused if you have mutliple boards attached, especially if you want each to have the serial monitor open! Each IDE instance should be self contained, and if I set the board type on one it shouldn't effect the other at all.

Which operating system are you using?

How are you creating the simultaneous instances?

What do you mean by "IDE gets confused"?

Is this issue restricted to the latest release of the Arduino IDE (1.8.13), or does it also occur with the previous versions?

I can confirm this bug.
How to reproduce:
a) Have two (different) arduino boards (example: Arduino Nano / Adafruit 328).
b) Connect them both to a PC with USB serial (using two COM ports on the PC).
c) Launch the arduino IDE, load the project, then File|Open|Select another directory and file will open a new window and load the other project, keeping the old one open still.
There should now be two arduino IDEs running each with their own project opened.
d) Choose board type and serial port on each arduino IDE and do the upload.

When this is done, then both arduino IDE’s are set to the same board type and the same COM port, even though something different was selected.

This is highly annoying and cost me (or the customer) some hours of development time.
This is clearly a bug that needs fixing, it should be fairly easy.
It really spoils the point of having two IDE’s running.
It would be nice just to click the “upload” button having set the port and board type once and for the entire session. It has to be set every single time for every single upload.
Otherwise it is uploading to the wrong board or produces errors because the board type is wrong.

While you’re at it, can you please do more to show the full path of the opened files?
Hovering the mouse over the editor tab should show a balloon text with the full path name.

The operating system on the PC is Windows 10.
The arduino IDE is 1.8.13.
I’m using external editor (Microsoft visual studio code).

christianhildebrandtnielsen: c) Launch the arduino IDE, load the project, then File|Open|Select another directory and file will open a new window and load the other project, keeping the old one open still.

OK, but that's not creating two instances of the Arduino IDE. That's creating two windows of the same instance. This happens when you use File > New|Open|Examples. The behavior you describe has always happened when you are using multiple windows of the same instance.

The solution is to open separate instances of the Arduino IDE by starting the Arduino IDE multiple times. When you do that, you should not find that you can have separate boards, ports selected, and even a Serial Monitor windows open to each port.

christianhildebrandtnielsen: While you're at it, can you please do more to show the full path of the opened files? Hovering the mouse over the editor tab should show a balloon text with the full path name.

This functionality is provided by the alpha stage Arduino Pro IDE. In addition, the Pro IDE allows you to have multiple boards, ports, Serial Monitors selected simultaneously even if you only use File > Open|New to create the other windows.

So, even if the trusty old Arduino IDE isn't exactly up to your specs, at least Arduino seems to be moving in the direction you want!