If your second PR is ready before the first PR is merged, then two of the likliest outcomes are that either PR reviews are taking too long, or the second PR is small enough that it could have just been part of the first. Alternatively, the review is taking a long time because the first PR was bad/controversial, in which case the assumptions of the second PR might need to be reevaluated anyway.
Neither of those cases need to be true for a second PR to be ready before the first has been merged.
For example you do your first PR, mark it ready for review. While doing it you notice there's some refactoring you could do to some tangentially related code. It's very conceivable that the second refactoring PR could be ready pretty quickly.