That's a rookie mistake.
Was able to book multiple cars. Picked up 1st car with AU in passenger seat next to me. Checked out at a national location. Drove to a second National location. Picked up second car. First location called and requested to return one of the cars because the rule is you can only get one car.
+1
Odd. How would the first location even know the second location reservation system. You still haven't mentioned the location/locations? Wondering if they were corporate or franchised.
Other's have had success so it isn't just me. National customer service also confirmed you can have more than one car out at the same time, though I would never try more than 2 at once.
The only other thing I can think of which might be a factor is possibly insurance? I've had full insurance on the cars when I've done it.