If you want to merge two lists, follow these steps:
1. Navigate to 'Contacts > Lists' in the main menu. Click on the list you want to add contacts from to another list.
2. Click on 'Edit all'.
3. Click on 'Add to a list' and select the list to which you want to add the contacts.
4. If you also want to remove the contacts from the current list, select the list under 'Unsubscribe from a list'.
5. Click on 'Apply'.