The problem with this is that it could potentially disqualify you from whatever rate you have obtained, and that will lead to a nasty surprise on return.
I would be inclined to make two reservations, but call and see what your options might be.
Just to clarify on the no-penalty part. Make sure you read the fine print in booking, some higher end vehicles (ie. Hertz Prestige) and even minivans have been seen with "cancellation" charges, if cancelled within certain number of hours/days.