The liability that is imposed there is known as joint and several which means that the plaintiff could sue me alone or could sue the employer alone or could sue both of us and whatever judgment the plaintiff gets against us could then be collected by the plaintiff against me solely or against the employer solely or against either one of us to the extent that we have assets to pay. Under the principle of joint and several liability each defendant is 100% liable for the judgment that is rendered. That is a principle that has been under a good deal of attack lately because it can create circumstances wherein a defendant can wind up paying more than his fair share of any judgment especially if the other defendant cannot afford to pay.
Joint and several liability is something that is well ingrained into our legal system and the rationale behind it is to make sure that the Plaintiff can obtain at least one full recovery of whatever judgment is entered. It then becomes the burden of those defendants against whom the judgment has been entered to fight among themselves as to any eventual sharing of that liability.
As to this personal injury concept and others see the pages on this site for a detailed explanation of tort principles. For more information about joint and several liability see the pages on Wikipedia.