The iODBC driver manager is provided dual-licensed under the terms of both the Library GNU Public License Version 2, dated June 1991, or the New BSD license, at your choice.

Source

BSD is the more flexible license. Do we need both for the dual license case?

No we don't need it per se but the fact that we have a list for meta.license allows us to expose both to Nix/NixOS users and let them choose which one suits their requirements.
And I think this is what might be intended upstream with a dual licensing.

The meaning of a list of licenses is not clearly defined. One could interpret it as dual licensing ("pick one") or the collection different licenses used within the package ("all apply", what it means in the majority of cases). To make it even more complicated: a lot of libraries come with a set tools and the library has for example a LGPL license and the tools have a GPL license.
There is no way of telling what the list means just by looking at meta.license alone.

The reason why I picked one was an earlier discussion with a similar dual licensing constellation: #45282 (comment)

Ah thanks for the link to the previous discussion.
Indeed the interpretation of the meta.licenses list remains vague. Is it OR, AND, something else?
Myself I read it as "here is all the licenses that applies to parts or totality of this project".
Then it's up to the user to dig deeper if he needs/wants to.

But just like joachifm in the previous PR, I was just raising a question but I'm fine either way. There is no definitive good/bad policy there. :)

I don't have a strong opinion on this topic but I would tend to stay with picking the more flexible license for the sake of simplicity.