Why does account B need to be backed up?
You don't need to backup both accounts on separate drives. You can use one, but you have to be careful that you don't mix up your steps.
Let's say that you want to a Bullcrappie Drivercar GT but don't want to lose half a million buying it, so you decide this is the car you want for free today. You have two accounts, which I will call "Adam" and "Bob" (A and B, you see). Adam is your primary account and Bob is the one you use for your duping.
First, backup Adam's save file to your USB drive. Then load up the game as Adam and buy your Bullcrappie Drivercar GT. Send the Bullcrappie Drivercar GT to Bob, but
don't switch users to play as Bob yet. Exit GT5 and then transfer your USB backup back to your PS3, effectively giving you your half million bucks back, but without the Bullcrappie Drivercar GT. That's fine because it's still in the network on its way to Bob. Once it's been gifted to Bob, your save file cares nothing about what became of that car. The network has it.
Now, after you're restored Adam's save file back to your PS3, switch users to Bob.
Don't back up his file yet. Wait until
after you load the game and receive the Bullcrappie Drivercar GT. Once you've done this, you can either just gift it straight back to Adam, which will get you your car at no cost to you,
or you could keep it on both accounts. To do this, leave the game
before gifting the car back to Adam. Backup Bob's save file to your USB drive. Load the game back up and then gift the Bullcrappie Drivercar GT to Adam. Exit the game and restore Bob's save file to your PS3.
Now Adam has a free Bullcrappie Drivercar GT, and if you took my extra steps you have another one on Bob's profile as well, all free to both accounts.
An alternative method is to get Bob a bunch of credits and just backup his file to USB. Have Bob buy the car you want, gift it to Adam, and then restore Bob's backup file back to your PS3, giving Bob his money back. This requires that Bob have sufficient spending money, of course.
Note that I'm not encouraging players to do this. However, it's your game and if this brings you happiness then this is how you do it.