Personal Excess Liability Insurance extends one line of coverage 

Personal Excess Liability Insurance provides additional coverage when a single, basic liability policy has reached its limits. Think of it as an extension or rider to one of your policies. For example, if you have a Personal Excess Liability policy on your auto insurance, it can only cover an auto accident claim that goes beyond your auto policy limits.  

Personal Umbrella Insurance covers you across policies 

Personal Umbrella Insurance is a type of excess insurance that applies to multiple basic liability policies simultaneously, and may even cover additional claims not included in these policies. For example, a personal umbrella policy may cover you overseas, while most basic policies do not.