How to Bind List of Values together so when either parameter is select the other auto populates

Summary:

Wondering how to bind list of values together so that when a user selects one parameter, then the other is auto populated and then vice versa. So far all I can do it link one side and cannot get it so that it works when either information is populated.

Content (please ensure you mask any confidential information):

I am trying to link username and person number together so that if someone enters either of these in the parameter then the other will auto populate. I have only been able to get it to work with only one side binded. I would like to use the parameters as an OR condition so that if person number is entered then username is auto populated OR if username is entered then person number is auto populated.