user30341: Is it possible to monitor multiple USB ports simultaneously in Arduino IDE while running only ONE instance of the IDE?
If not, which solutions come alternatively to mind?

user49126: Short answer : No. 
Long answer : Noooooooooooooooooooooooooooooooooooooo, since one window can only work with one arduino. 
That being said, if you open another file, or create new file (Ctrl + N) on existing window, it doesn't make another process. 
It is still on the only javaw.exe process running on your computer (you kill that, both window gone for good.)
To make it simple : 

Double click Arduino icon on your desktop, and double click it again, it will run as 2 separate process. 
Double click Arduino icon on your desktop, and then use open file or new file, it won't make another process. 

But I think I misunderstood your question. If you're asking for arduinos in multiple USB ports, yeah, one IDE can see wherever you plug it in. 

user19867: Sloeber, an open source IDE based on Eclipse on which I am a proud contributor, does have this feature, but the Arduino IDE does not.

user41696: If you are only interested in monitoring the output, a simply python script can be run to monitor the port. This sample takes an input of the port number(assuming they get named ttyUSB1, ttyUSB2, ect...  Or you can modify it to be a hard coded port.  Also make sure the baud rate matches with what your arduino is running (115200 in the example)
You can then run this script in two separate terminals, one for each port
#!/usr/bin/env python3


import serial 
import sys

if len(sys.argv)<2:
    print("Please give serial port number")
    exit()
port=sys.argv[1]
ser= serial.Serial('/dev/ttyUSB'+port,115200,timeout=5)

while True:
    try:
        print(str(ser.readline().strip()))
    except:
        print(sys.exc_info())
        ser.close()
        exit()

Start the program like so 
python3 ./programName.py 1 (or ./programName.py 0, etc)
The number dictates which of the ttyUSBx devices you are trying to watch (will match with what you see in the arduino IDE).

user37622: You can run multiple instances of the Arduino IDE to work with multiple boards at the same time. Start IDE second time with Start menu or desktop shortcut.
Of course you can monitor the COM/tty port with a common serial terminal program too.

30341: <p>Is it possible to monitor multiple USB ports simultaneously in Arduino IDE while running only ONE instance of the IDE?</p>

<p>If not, which solutions come alternatively to mind?</p>
 49126: <p>Short answer : No. </p>

<p>Long answer : Noooooooooooooooooooooooooooooooooooooo, since one window can only work with one arduino. <br />
That being said, if you open another file, or create new file (Ctrl + N) on existing window, it doesn't make another process. <br />
It is still on the only javaw.exe process running on your computer (you kill that, both window gone for good.)<br /><br />
To make it simple : </p>

<ul>
<li>Double click Arduino icon on your desktop, and double click it again, it will run as 2 separate process. </li>
<li>Double click Arduino icon on your desktop, and then use open file or new file, it won't make another process. </li>
</ul>

<p>But I think I misunderstood your question. If you're asking for arduinos in multiple USB ports, yeah, one IDE can see wherever you plug it in. </p>
 19867: <p><a href="http://sloeber.io" rel="nofollow noreferrer">Sloeber</a>, an open source IDE based on Eclipse on which I am a proud contributor, does have this feature, but <strong>the Arduino IDE does not</strong>.</p>
 41696: <p>If you are only interested in monitoring the output, a simply python script can be run to monitor the port. This sample takes an input of the port number(assuming they get named ttyUSB1, ttyUSB2, ect...  Or you can modify it to be a hard coded port.  Also make sure the baud rate matches with what your arduino is running (115200 in the example)</p>

<p>You can then run this script in two separate terminals, one for each port</p>

<pre><code>#!/usr/bin/env python3


import serial 
import sys

if len(sys.argv)&lt;2:
    print("Please give serial port number")
    exit()
port=sys.argv[1]
ser= serial.Serial('/dev/ttyUSB'+port,115200,timeout=5)

while True:
    try:
        print(str(ser.readline().strip()))
    except:
        print(sys.exc_info())
        ser.close()
        exit()
</code></pre>

<p>Start the program like so </p>

<p><code>python3 ./programName.py 1</code> (or <code>./programName.py 0</code>, etc)</p>

<p>The number dictates which of the ttyUSBx devices you are trying to watch (will match with what you see in the arduino IDE).</p>
 37622: <p>You can run multiple instances of the Arduino IDE to work with multiple boards at the same time. Start IDE second time with Start menu or desktop shortcut.</p>

<p>Of course you can monitor the COM/tty port with a common serial terminal program too.</p>
