You would definitely have to have them both under your name if you want to take advantage of your President's Circle benefits, and yes, you would have to pay an additional driver fee, unless you booked with a CDP like AAA or USAA that waives the fee. I'm not even 100% certain Hertz will allow you to have two vehicles checked out at the same time under the same name/account.

All of that said, there is a much better/easier way to approach this which is to have your father-in-law sign up for Hertz Gold and then use his benefits. Feel free to PM us and we can give you some pointers on this on how to do this in the optimal way.