Actually, we do have one overlapping FP+ in my family's upcoming plans, but I suspect it's fairly uncommon. I was scheduling Anna and Elsa one person at a time for our family of four (which was the only way to get any A&E times for us at all), and as I copied my FP+ itinerary to my DD, the A&E time that was available for her was overlapping 10 minutes with one of her (our) other rides. I think the system will not initially offer you overlapping FP+, and it may be that the only way it will happen is if you're copying from one party member to another and one of the attractions only has a different (but close) time available for the second person.