RPi wont boot RatOS when 2 USB boards connected

Hi Everyone.

Last night I put RatOS on SD card and booted the RPi. During the setup I connected (via usb cable) the BTT Octopus MAX EZ board and flashed, then connected the EBB42 (via USB cable) and flashed. All seemed to be going well and was able to do the printer.cfg config. Once done I shutdown and went to bed.

Today I tried switching on the system again and I first switched on the BTT boards (the MCU's have a separate 24-volt PSU and the RPI its own 5-volt PSU), then switched on the RPi only to notice that the RPi power light comes on but green light never flashes and thus not booting.

Removed both USB cables... RPi boots,
Add 1 MCU.... RPi boots,
Add 2nd MCU.... no boot,
remove first MCU... RPi boots.

So not specific to the USB MCU as it will boot with either plugged in, but not with both.

Tried another RPi-specific PSU and nothing... Connected to the first PSU and bumped voltage to 5.2V... Still nothing...

Has anyone seen this before? Any Advise?

Edit:
Just to add, while both USB cables are connected and the power is off on the MCU's, RPi will boot and I can power on the MCU's afterwards and everything is working.
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