Depending on your need you could easily use CSJS or SSJS for this.
create an onChange event of the Employee ID field with the script:
SSJS:
getComponent("username").setValue(getComponent("employeeID").getValue());
CSJS:
dojo.byId('#{id:userName}').innerHTML (or .value) = dojo.byId('#{id:employeeID}').innerHTML (or .value)