Hi @ash
I’d strongly recommend having each OS on separate disks, much as @Wollie has suggested.
You could install Manjaro to the new disk and remove the previous Manjaro partitions from the original disk (after copying your /home content somewhere safe, if needed).
Once Manjaro (or at least the Manjaro UEFI boot files) are removed from the original disk, you could install Manjaro to the new disk.
The following guide may be helpful; skip the installing Windows steps, as you already have it installed:
Or, if you still prefer to simply clone the existing disk, the following guide may be helpful:
Regards: