iCloud doesn’t give you much flexibility, but it might not matter for this. I think as long as the lower vault has “.nosync” at the end of its name, iCloud will ignore it (that might be outdated info).
I think the problem is that Obsidian Sync won’t sync the lower vault. The configuration that enables it is in a vault that is not currently open (Obsidian Mobile can’t open multiple vaults simultaneously). This is if I’m understanding right that I would be interacting with the upper vault (so I can access the attachments folder and the lower vault together).
I’ll try it with a test vault anyway in case I’ve misunderstood or am wrong about how Obsidian works. Good thinking, either way!
Oh, i just thought of another problem: Obsidian movbile vaults currently have to reside in Obsidian’s dedicated folder. I think that can either be on the device or in iCloud, but I’m not sure if you can use Sync if it’s in iCloud. Anyway, I’ll try it when I have some time.