I wanted to setup a raid1 configuration with btrfs and two external 2.5" drives. Error as described here. And you’re right - it seems that the rpi 4 cannot power 2 usb drives, as it is working with one 2.5" and one 3.5" drive (3.5 with external power supply). Just tried now.
Well, it seems we have to accept that fact - wondering why the original power supply delivers 3A, while the device can’t even support 2 usb drives?