I have two overlapping polygon layers and I'd like to display the labels from the bottom layer on top of both layers so they're easily visible. Is there a way to do this with CartoCSS? Some sort of z-index for the labels?
You could add the bottom layer again, on top of the top layer, and then make then change the polygon's opacity to 0:
Then configure the infowindows for the new top layer, and disable infowindows for the bottom two layers.
Hope that helps :)