You CAN practice. If you have tickets linked to
MDE you can make FP+ reservations 30 days out, even for days that are not really part of your trip. I am staying on site and my 60-day FP+ window for our actual trip doesn't open until Sunday, but I've been able to make selections for any date within the next 30 days (I'm sure this is limited to the number of days on my ticket, but I've only tried to do a day or two for practice). You can do this for practice, and then go cancel your selections so as not to take slots from people who will really be there those days.
To copy selections, go back to the main FP+ menu, select view or modify existing selections, then select copy from one party member to another.