OK I managed to downgrade from FW5.1 to ACKMANIAC FW.
Here’s the way to do it, with bricked DUAL (V)ESC :
- Upload latest FW (5.1 nowadays) (https://github.com/vedderb/vesc_tool/tree/master/res/firmwares) to both STM with ST-LINK utility.
- With vesc_tool_2.5 : Firmware >> Bootloader >> “(upload) All” button.
- Write motor configuration, then Write app configuration, then Disconnect.
- With ACKMANIAC-ESC-TOOL : upload firmware to each (V)ESC.
You’re done! 
Official FW is not bad, ACKMANIAC is just better IMHO!!!
I didn’t get to recover USB port on repaired ESC side. It seems dead. I really wonder why…