You don't have to unlock your other system destination.With them both using the same key, you'd have to untarget, jump into supercruise, fly around the planet enough, then retarget and jump. But with two different keys, you keep the target and use the supercruise key to get around the obstacle.
Keep it locked, then you select something inside current system, but it doesn't cancel your lock on next system.
What you need though is to bind the key that will select your next destination (it's not bound by default).
So I select system to jump to, there is obstacle on the way, I select some station/planet in current system, supercruise towards it and once the original "next system" path is clear, I press space (that's what my binding is) to select next system and initiate FSD.