I typically use mix cmd --app umbrella_child mix run or pushd/popd for that, together with Tmux splits/panes, which can optionally be automated with projects like tmuxinator, teamocil, or a recent entrant called dmux.
Our primary repository at work is a large umbrella where most development efforts target one web frontend plus data and batch processing, so around a quarter of the total repo footprint, and this model works well in that scenario.
