When you arrive at the site of Rail Europe, you will see the search panel at the top of the screen. Input Paris as the departure point and Carentan as the arrival point. Be sure to select the correct date for your journey and set the departure time as 07am. Next to the selection box where you have just entered the time, you will see an icon to add another trip. Click this and another row of selection boxes will appear.
In the new set of selection boxes, simply do the same as before but this time, enter Bayeux as the departure point and Paris as the arrival point. For the departure time, select 6pm. Once you have done this, click the SEARCH button.