Bonjour! We are arriving at CDG and leaving France from Orly a few days later. When I purchase the ticket: CDG to St.Michel, can I also purchase a ticket: St.Michel to Orly at the same time? 2nd Question: Can I purchase these RER-B tickets in advance online?
Yes, you can purchase both RER B tickets (CDG to St. Michel-Notre Dame, St. Michel ND to Orly) at the same time upon arrival. Easiest to buy the Paris to Orly RER B tickets at CDG train station ticket windows from an agent so they can customize the start/end stations for you. Otherwise you'll be hunting through options on an automated ticket machine trying to create that ticket yourself.
Unfortunately RER B tickets are not sold online. You'll need to buy those upon arrival at CDG, at either T2 or T1/T3 train stations.