I assume you're batch sparging? Just do a double sparge. Your mash water will definitely fit--it's the sparge water that might not. I'm actually brewing a 10-gallon batch with my 10-gallon cooler as we speak (it's on the burner now).
Here's what my water looked like: Mash: 4.5 gal, Sparge: 10 gal, Pre-boil total: 12.38.
All I did was split the sparge water, and everything went fine. Added 5 gallons twice, and that brought me about an inch below the brim on each one.