Which does the same job as grub … maybe thats why you think there are “2 grub menues” ?
I have no idea why refind is mentioned so often.
There was a time in years back when refit or refind was really the only way to boot something other than macos on macs … but beyond that I really dont understand why its ever used (or talked about even).
But I greatly recommend to keep grub and get rid of refind (specially when you are not sure what are you doing)
About booting Windows, it seems that the Windows bootloader in the EFI partition is just the remainings of an old Windows instalation in that disk. You can remove this with:
sudo rm -rf /boot/efi/EFI/Microsoft/
And then update your grub menu with:
Your Windows installation in /dev/sdb is installed in BIOS mode. Unfortunately you can’t boot a BIOS installation from a UEFI grub, so you’ll have to keep booting Windows the same way. Only options are to reinstall Windows in UEFI mode or reinstall Manjaro in BIOS mode.