Charles de Gaulle Airport to Orly Airport...
You can go by...
Charles de Gaulle and Orly airports are on different sides of the city with three realistic options (don't drive!) for transferring between them.
1) Transferring between Paris CDG airport and Paris Orly airport is easy...and free if you are an Air France passenger in transit. The Air France Line 3 shuttle coaches depart every 20 to 30 minutes from 5:55 am to 9:00 pm, 7 days per week and the journey between airports takes about an hour. It departs from Paris de Gaulle terminal 1 at exit 34 and terminal 2, halls A, B, C, D and F. Fare for non Air France passengers is € 20 each way for adults and € 10.00 for children.
2) The second practical way to make the transfer, especially if you are 'travelling light' is my using the RER train service from within Charles de Gaulle Airport, combined with a short trip on the OrlyVal shuttle train to Orly airport itself. This service takes you through the centre of Paris - you stay on until the station named 'Antony' where you pick up the OrlyVal shuttle. The combined fair is € 19.65 for adults and € 11.70 for children.
This is perhaps less convenient than the coach as there, they handle your luggage, whereas you have to take it through a few turnstiles and up and down stairs on the train trip.
3) The third option is by taxi if money is no problem but for the average traveller one of the other options might be better. This also applies to the shuttle services which allow you to keep the price down by sharing with fellow travellers or privately for a group. Expect to pay around € 70.00 for a taxi and more for shuttle service. It is advisable to get your fare confirmed before entering the taxi. Try and have euros available to pay as few taxis allow payment by credit card.