Ok, this is so wierd 
Did you try loading any other older fw? Maybe 3.38 or something you have at home?
No I don’t have anything else than the one installed…
Oh, yes I’ve tried with an older version of VESC Tool
You need to use one ESC as a programmer to flash the other. Simply connect both SWDs together (GND IO and CLK only) GND to GND, IO to IO, CLK to CLK. Cables should be short, 6-8cm only!
Then you connect to the working ESC ( the one with the latest FW on) via VESC-Tool and open the SWD PROG tab. Now you flash the non working ESC with the 410,411,412 file.
Easy!
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That’s a cool way to do it. so for people that don’t have st link they just need to make a cable if they have a dual setup.
I would have thought that without the bootloader the esc won’t even connect to the PC though…
Awesome! I will try this after work! 
don’t spin a motor while you are connected!
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It’s a little hack for your convenience. A present from Benjamin to the community. It was not easy to implement but is very beneficial.
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AMESOME! It worked Thank you so much! This was a great fix! 
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You are welcome!
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could you post a tutorial of this? im a having the same issue
Just follow the steps Trampa posted above! 
Both VESCs should be powerd by a battery, so that both STM processors are up and running. Both VESCs are interconnected SWD to SWD ( GND, IO and CLK only)
Open the SWD PROG tab (menu on the left at the bottom), hit connect and then you select the VESC you have and click upload. Programming has never been easier! Make sure not to choose the wrong hardware! When the upload has been done, you wait for 10 seconds and disconnect everything and then you are good to give things a try.
Why the 410, 411, 412 file? I think my VESC 6.6 dual came with 60 on it. Or at least thats what has always been on it for me. Irregardless that I am having major issues with cogging and not being able to get both sides of the VESC to run properly. It’s maddening…
What does flashing the bootloader do? Is something corrupt maybe?
Since I’m using VESC 4.12
Quick technical question here: Trampa indicates that the SWD cable should be 6-8cm in length…why? Is there too much internal resistance in the wire?
Edit: Actually, that question was for anyone/everyone if they have any input…
Much oblige mates
Longer cables cause problems on SWD.
This is an explanation for JTAG, which is similar to SWD when it comes to max cable lengths. http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0517b/Chdbgcdj.html
Still having issues, soooo when you say disconnect everything else, do you mean the 3 motor wires too (non-sensored motors so hall sensors wires N/A)? Remote or bluetooth (if we have one)?
We need to have at least one USB data cable connected to the good VESC of course, right?