Hot and cold are distinctions to allow you to save money on storage since in theory cold doesn't need to have the performance that hot does. You can certainly use one VMAX filesystem for it, but that's fairly expensive storage. If you can make a second lower performance filesystem (lower redundancy, slower disks, etc), then you can save on the cost per TB. Other than that, no reason not to put them on the same filesystem.
If you do send it to one filesystem, don't make two separate mounts, as the OS will end up copying files from one to the other, since it will not know it's the same filesystem. If it's one mount, it's just a pointer move when a bucket rolls from warm to cold.
