I couldn't wrap my head around it either...I was thinking I was supposed to hit ctrl+c and then put it in.
When you make your FP+, you will choose the desires for one or more of your party. If everyone wants the same attractions, select all when you begin. Children under the age of 7 will have to have their attractions chosen with an adult.
After you make your FP+ and you have had them confirmed...you can go back into the system and click on "Change..." There you will see buttons for Replace, Change time, Copy or Cancel. Click Copy. Then you choose the person who has the FP+ you want to be the same. The next page will ask who to copy to. And then the system will check and see if it is possible. All going well, you will receive a confirmation. If not, you will see "this is not available...alternative has been chosen
I hope this helps.