My configuration has a raspberry pi 3 (ubuntu mate 1.16.2) with two arduino nano connected via USB.
When the two arduinos are unplugged from their circuits, upload works fine. When they are plugged, upload fails with
Arduino: 1.8.5 (Linux), Board: "Arduino Nano, ATmega328P"
Sketch uses 5668 bytes (18%) of program storage space. Maximum is 30720 bytes.
Global variables use 618 bytes (30%) of dynamic memory, leaving 1430 bytes for local variables. Maximum is 2048 bytes.
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x00
Both boards have TX, RX and RST pins not connected in the circuit.
Almost every I/O pin is used, Each board loads a different sketch and it is connected to the circuitry with a different schematic.
When the system is running and boards are plugged, both boards work properly.