How can I wake up/turn on Linux powered PC from another Linux powered PC via mutual USB cable?
The best way to actually do this might be via two USB ethernet cards and WakeOnLan.
PC - USB_NIC ---------(ethernet cable) ---------- non-USB_NIC -PC to be woken up.
Which only works one way and which seriously deviates from USB.
Direct USB to USB is propably not an option because USB is split into master and slave. (This is why USB cables have two different headings; one plug for masters, one plug for slaves. This prevents you from plugging in two masters or two slaves on a cable. And it is not just a matter of changing the cable, the actual USB chipset and drivers need to understand their part and only a few of them are capable or both master and slave mode)