I don't think you quite understand the difference between non-generic and generic case.
List<A> has no (inheritance) relationship to List<B> regardless of (inheritance) relationship between A and B.

However, if I understand your reasoning, the motive would be to invoke some behavior on the type contained in the list regardless of what is in the list.
If that is the case, you can (and should) refactor that behavior into an interface (let's call it C) and then derive A and B from C. Further, you should always use a List<C>.

Alternatively, you can use a common base class C instead of an interface.