Sample code to show the sequence in which observable-redux dispatched events are reduced.

observable-redux-action-timing.js

import { createStore, applyMiddleware, compose, combineReducers } from 'redux';
import { createEpicMiddleware, combineEpics } from 'redux-observable';

const makeActionCreator = (type, ...argNames) => {
    return function (...args) {
        console.log('Creating action', type, args[0]);
        const action = { type };
        argNames.forEach((arg, index) => {
            action[argNames[index]] = args[index];
        });
        return action;
    };
}

enum Actions {
    ACTION_1 = 'ACTION_1',
    ACTION_2 = 'ACTION_2',
    ACTION_3 = 'ACTION_3',
    ACTION_1B = 'ACTION_1b',
    ACTION_2B = 'ACTION_2b',
    ACTION_3B = 'ACTION_3b'
}

const dispatchAction1 = makeActionCreator(Actions.ACTION_1, "value");
const dispatchAction2 = makeActionCreator(Actions.ACTION_2, "value");
const dispatchAction3 = makeActionCreator(Actions.ACTION_3, "value");
const dispatchAction1b = makeActionCreator(Actions.ACTION_1B, "value");
const dispatchAction2b = makeActionCreator(Actions.ACTION_2B, "value");
const dispatchAction3b = makeActionCreator(Actions.ACTION_3B, "value");

const testReducer = (state = {}, action) => {

    switch (action.type) {
        case Actions.ACTION_1:
            console.log('Reducing action 1');
            return {
                ...state,
                value1: action.value
            }
        case Actions.ACTION_2:
            console.log('Reducing action 2');
            return {
                ...state,
                value2: action.value
            }
        case Actions.ACTION_3:
            console.log('Reducing action 3');
            return {
                ...state,
                value3: action.value
            }
        case Actions.ACTION_1B:
            console.log('Reducing action 1b');
            return {
                ...state,
                value1b: action.value
            }
        case Actions.ACTION_2B:
            console.log('Reducing action 2b');
            return {
                ...state,
                value2b: action.value
            }
        case Actions.ACTION_3B:
            console.log('Reducing action 3b');
            return {
                ...state,
                value3b: action.value
            }
    }
    return state;
}

const action1Epic = (action$, store) =>
    action$.ofType(Actions.ACTION_1)
    .mergeMap(action => [
        dispatchAction2("From 1"),
        dispatchAction3("From 1")
    ]);

const action2Epic = (action$, store) =>
    action$.ofType(Actions.ACTION_2)
    .mergeMap(action => [
        dispatchAction1b("From 2"),
        dispatchAction2b("From 2"),
        dispatchAction3b("From 2")
    ]);


const testEpics = combineEpics(action1Epic, action2Epic);

const epicMiddleware = createEpicMiddleware(testEpics);
const testStore = createStore(
    testReducer,
    applyMiddleware(epicMiddleware)
);

// Dispatch the first action
testStore.dispatch(dispatchAction1("From the top."));