I don't know if there's a specification or formula guidelines to create specific search querys, and the only way I can think of doing this is to change the two fields names to a single name so that they'll be the same field for QlikView. That way when you search and select a name in that field it will affect all the document sheets and info. Although this way the field will become a key and you'll have to think how it may affect your data model.
However, now that I think of it, you say that the design for both databases is the same, do you mean that the databases tables are similar? If so, why don't you concatenate the tables?
What you really should do is rename one of the two fields so the both get the same name. That way the tables can be associated and you will only need to search in one field.
If for some reason you don't want to do this you can try using adding Select in Field actions to the OnSelect field triggers of the two fields. You'll need something like this expression as search string for the Customer field: ='(' & concat(distinct chr(34) & CustomerName & chr(34), '|') & ')'.