It appears that a 'request changes' review blocks merge even if another core-dev approves. I'm not sure whether two approvals would override a 'request changes', but it is possible to dismiss a review; we should decide criteria for dismissing another core-dev's review here and document it in the devguide.

For example, python/cpython#27 was blocked by my outdated review, even though @methane had made the changes I had requested and @ncoghlan had approved the change. Frankly, I had forgotten that I had reviewed that PR, so I would have had no problem with either of the other involved core-devs dismissing my review.