Bedouin, if you want the customername in the grid when you bind table2 collection to the grid, you have to map a field on a related field in table2's entity as Jeff explained. This then at runtime makes 'table1.customername' part of the columns of the grid when you bind the table2 collection, as every table2 object in the table2 collection then has an extra field, customername. This is necessary because 'customername' is part of another entity, not part of table2.
If you just want to display a list, (readonly) you can also use a typedlist: add Table1 and Table2 to the typed list, and select the fields you want, for example the table2 fields and also table1.customername. Then fill that typedlist and bind it to the grid.