You can clone with
From #1214, it might make sense for larger projects, and possibly TileMill as a whole, to have datasource settings that can be shared across layer configuration, whether verbatim or with changes like query contents.
Yep, I see one task here being how we want MML to work moving forward -- it has previously been a (more or less) direct mapping to Mapnik XML, and as Dane points out we'll need to come up with some stronger concepts to frame the format going forward if we want to handle these kinds of abstractions.
A good first step could be to looking into parsing layers separately from styles. All the necessary bindings are in place in node Mapnik to skip XML as far as layers go. Then it would be easier to set up event callbacks so individual layers could be updated without reloading the style sheet.
good discussion and further ideas at gravitystorm/openstreetmap-carto#4