Throwing another possible workaround to this dilemma into the mix, another option is to install the refind boot manager to act as the initial boot loader, chainloading both Manjaro and Ubuntu versions of Grub.
This suggestion only applies to a fully UEFI booting system.
refind also has a welcome feature of being able to bypass Grub and boot the kernel stub directly, which is convenient in the case that a problem with Grub prevents booting – one can still boot to the Linux system and work the problem in relative comfort.
I mention refind briefly in a related tutorial.
This does take some manual configuration, so I’d suggest spending the time needed to research, before taking this route.