From years in the past at the Hilton, we've had to pay double parking, but were able to get both cars at the Hilton rate.
You might be able to ask them if you can park in the regular convention center garage (technically the same building, but blocked off from the Hilton portion) at the Hilton rate if their garage fills up.
It's getting increasingly difficult to find parking there, it seems, but Friday afternoon is a good timeframe for when it fills up. It was already mostly full Friday morning last year when I parked (and managed to somehow snag a really great spot right next to the elevators). And don't move your car all weekend, since there's no way you're getting your spot back.
Don't listen to me. I might be wrong.
I remember last year having to meet my sister out in front of the convention center to give her my parking ticket so she could get her car out...so we may have only been charged once for parking. I can't remember. Either way, we were able to have two cars in the garage for one room.