If you have an application that consists of multiple components that work together or depend on each other (e.g. a service backed by a database) and you want to define relationship between these components on Marathon, you can use Application Groups: https://mesosphere.github.io/marathon/docs/application-groups.html
On the other hand, if you have an application that requires services to be co-located on the same host (e.g. a main process along with a sidekick log reaper process), you can package them into a single Docker container to be distributed and run together. We are looking into supporting Kubernetes’ Pod like functionality on Marathon in the future.