It is definitely ok to make muliple car rental bookings. Many people do this, (myself
included!), especially when
Just make sure that you aware
The only thing that I would ask of you, is, if you can, cancel the reservations that you make that you will not be using.
Cancelling the rentals that you know you will not pick up, allows the car rental companies to manage their fleets better. If a company has most of their cars reserved, the price goes up on the ones that are remaining, and if all the cars are reserved, people may be turned down for rentals because the companies are holding vehicles for people who have made rental car bookings, but who never show up. It's not something that you have to do, but cancelling, if you can, makes it easier for the rental companies and easier on your fellow travelers.
Pages from this site, relating to this topic:
Rental Car Reservation Basics
Using Booking Sites to Rent a Car