Is ULS dual licensed MIT and GPL?

Yes.

Looking at https://spdx.org/ids-how - I think the question to answer would it be AND or OR type multi licensing. I'm not fully sure about the implications, but I think the intention is OR.

In T251643#6101869, @Nikerabbit wrote:

Looking at https://spdx.org/ids-how - I think the question to answer would it be AND or OR type multi licensing. I'm not fully sure about the implications, but I think the intention is OR.

I think so too..

// SPDX-License-Identifier: Apache-2.0 OR MIT
    The licensee may choose to use the file under either the Apache-2.0 license or the MIT license.
// SPDX-License-Identifier: Apache-2.0 AND MIT
    The file is subject to both the Apache-2.0 license and the MIT license.
    The licensee must comply with both licenses when using the file.

Change 593898 had a related patch set uploaded (by Reedy; owner: Reedy):
[mediawiki/extensions/UniversalLanguageSelector@master] Update license-name to include OR MIT

https://gerrit.wikimedia.org/r/593898

Patch not necessarily to be merged just yet... but gives something to play with for T251642

Though, as far as MW is concerned, it just links the lot, so that's fine

Just need to do something when you click through to Special:Version/License/UniversalLanguageSelector, as even though there's 2 LICENSE files, nothing is displayed

Change 593898 merged by jenkins-bot:
[mediawiki/extensions/UniversalLanguageSelector@master] Update license-name to include OR MIT

https://gerrit.wikimedia.org/r/593898