The whole REP is worth a read. I haven't updated r_l to work with the changes involving the earth frame, but I believe it conforms to the rest.
Note that you don't have to have a dual EKF setup. It's just that if you want your state estimate to come from fused data, rather than a single data source, then r_l can support that. Other approaches, such as letting amcl handle the map frame state estimate (and transform publication) by itself, or letting your wheel encoder odometry source (e.g., the differential drive controller) provide the odom frame state estimate/transform by itself, will also work.