Sorry to say guys, but it seems dual-boot of Reborn OS with another Arch distribution seems wrecked.
As my chat with @shivanandvp on Discors brought up, this dual-boot is never been tested, instead of Windows and Reborn.
My testings so far included RebornOS Budgie as base installation on a switchable SSD. I never tested anything other than Arch so far, so this part may be unharmed.
I added one after the other Manjaro, ArcoLinux Deepin, RebornOS Gnome. In all circumstances, I was not able to pick the other dual-boot distribution from Grub, as it only produced black screen without a prompt or error notice. The only issue-free start was with ArcoLinux Deepin, but it produced a second Grub or Efi option, which I was able to choose over BIOS [F8], which is quite weird to show up this way.
It seems, that RebornOS has a basic issue when it comes to dual-boot setup with another Arch distribution. I’ve read something similar somewhere in the Manjaro forums, but am not sure about.
Hopefully this post may get someone from the team’s attention in the bug section.