Two years ago I purchased two separate sets of day permits, for the same date, (one for a group of four and one for a group of three) after the May 1 release, to accommodate an anticipated number of hikers. I was only able to obtain permits for one selected set of the purchases, when I went to the Visitors Center before our hike. I was informed that I was allowed to reserve/purchase only one set of permits under my name as the group leader, for a specified date. The disallowed purchase was released to the day lottery at the Visitor Center. No refund- which was OK by me since my hiking party ended up being smaller in number.
I would be cautious regarding purchasing additional slots for a day in which you already have a confirmed reservation. I don't know how this would work if the alternate leader picked up the reservations.
Best to check with the Visitor Center.
