Yes, pragmatically we can do both for Julia packages intended for General. That is also what I would implement as the default.
But I do not think this is conceptually neutral. Once a project uses REUSE to describe file-level licensing, a conventional root LICENSE file becomes a coarse compatibility artifact. If LICENSES/ contains several license texts, a human reader may easily treat the root LICENSE file as the authoritative project license, even though the actual licensing state is described by SPDX headers, REUSE.toml, and LICENSES/.
In a fully REUSE-oriented setup with multiple licenses, I would rather expect a root LICENSE file to be a pointer, e.g.
LICENSE
"This project is REUSE-compliant.
See LICENSES/, SPDX headers, and REUSE.toml for actual licensing."
But I understand that this would not satisfy current General registry expectations.
A future RegistryCI check could in principle go further and check whether each distributed file has an OSI-approved licensing path, as sketched above. I understand that this is not likely to happen immediately, but it remains the technically cleaner model in my opinion.