How to populate the store and sequentially await return using Redux Observable?

I am attempting to use Redux Observable to call an action to fetch some data, wait for its return, then fetch some more data that relies on it.

I have an epic which populates a store from a fetch FetchTodos. This listens for the FETCH_TODOS action and then calls my todos API and populates {todos: [] } =

I also have a comments section in my store todoComments. However, I would like to only populate todoComments once FETCH_TODOS has returned and populated the store.

In imperative code, this might look like:

let todos = await api.get('/todos');
await dispatch("FETCH_TODO_COMPLETE", todos)
let firstId = getState().todos[0].id
let comments = await api.get(`/todos/${firstId}/comments')
await dispatch("FETCH_COMMENTS_COMPLETE", { todo_id: firstId, comments})

The closest I saw to this was this issue in the Redux Observable Repo, but I could not understand how to do this efficiently. This is a pretty common scenario for me.

I would like to reuse as much code as possible. In this example, I may dispatch FETCH_TODOS from multiple components.

How would i accomplish this with Redux-Observable?