To add multiple routes to one map you need to follow the following steps:

1. Open the route with which you wish to start

2. Select 'Routes' on the left side of your screen

3. Select 'add'

4. Select the route you wish to add

5. Select the 'plus' icon

6. Choose between adding the second route at the beginning or the end of the first route

7. Select 'Merge'

8. Your routes will now be merged