The walk-ins are free, and become available at that 10 AM time.
Calling several times a day during the week immediately prior would likely work. But the extra reservation will cost $15.
Here's a third option: Since you already have the reservation for two, when you go to pick up the permit (even the day before), you can request an additional spot. I am not sure about this, but I believe they might just give you the extra one then (as long as a cancellation is available). This would effectively put you at the front of the line. You might call the permit office and ask about this.
Read the "Wilderness Permit Options" (link in the Feature Topics on the left) for more info.
