The USB power controller in a Pi3/4 is a DIODES AP255X Precision Adjustable Current-Limited Power Switch???The USB Pi wiring is not included in the official Raspberry Pi schematic but the datasheet shows how the circuit is likely wired on the RPi.?The chip is configured for 1.2A max current for the USB 5V rail.??The USB 5V is common to all 4 ports. So the 1.2A is shared.? The AP255X provides reverse current and voltage protection.? It also asserts a FAULT condition to the processor.? I can find no information on the web about what happens for this situation.
As I stated in my previous post, I suspect that the Pi will be able to power two SHARI's simultaneously.? If not then the easiest solution is to use a powered USB hub.? The split cable powering approach should work if the USB power connectors are connected to the SHARI USB jack closest to the center and USB cables providing data only connection are connected between the PI and the outer USB connector on SHARI.? However, be aware that there is no overcurrent protection (i.e. fuse or current limiter) built into SHARI as that function is normally provided by the Pi USB port.
Steve-N8AR