Is it wrong is molre like a personal opinion. If you haven't held the car with a cc than you're safe. On the other hand, it's like reserving multiple PS's - you can by blocking someone else from getting it. Maybe nut just your one, but all of the other "ones".
Would I do it? Hmmm. Only if I were not so sure of the first rental company that had a deal that sounded too good to be true. In which case, it probably is. Sorry, just rambling a little bit playing both sides of the fence.