HowTo: Pre_write other fields amount to a single field

In HOW TO, http://www.xlinesoft.com/articles/amount_to_single_field.htm

Show how to write other fields amount a single field.
But I have another usage: Pre_write.

1. I have User table, with Name,Age,Address
   
2. I have Order table, with UserName,UserAge,UserAddress.
   I can use "Lookup table --> List page with search" to select UserName from table 1.
   
   But I want the field UserAge,UserAddress amount to Name ,auto pre_input in that field,
   
   and user can change it manual.
   How can I do this?