Just asking, is it possible to upload multiple Arduinos at once?
I think it's pretty possible though.
Not really important...
Well I have four Adruino Nano clones and when I try to upload the same code I wondered how to upload the code to all of them at once.
Thanks for solving my curiosity!
You could TRY connecting all four RX pins together but only the unit that has the TX pin connected will be properly verified.
Similarly you could try connecting the MOSI, Vcc, Ground, Reset, an SCK pins of an ISP Programmer to all four and connect MISO (Master In/Slave Out) to only one.
Considering the time to load a program, you could also make a small batch file and load them using AVRDude. It looks a little intimidating at first, but once you have done it it is rather easy and quick.