I had the same question and checked upon this with National. I'm Executive, big family, wanted to rent 2 cars at the same time. One for myself, 1 for my wife. See below the question and the response from National:

Question:

Good afternoon,

I'm an Executive EC member. Next year I'll be on holiday in Florida for 3 weeks with my family of (in total) 6 persons. This time we want to rent 2 cars. Question is: can I rent two cars on my Executive EC card, pick both of them from Executive Aisle and ride one car myself, the other one by my spouse?

Thanks for answering, kind regards.

Answer:

Thank you for your inquiry about your future rental. My name is Jeannette and will be happy to assist you. Yes, you may reserve two rentals in your name. If I can be of further assistance, please feel free to contact me.

Sincerely,
Jeannette
National Customer Service