You seems using look up for field "2".
If you place Source::B field on the layout, it show "B100" and editing it change Source::B value.
But Field "2" will stay "B100". you need re look up. Do you need field "2" having other value than Source::B sometime? If not, remove the field2 is simple answer.
If you need, for automating re look up, you can use script
Set Field [ Field1 ; Field1 ]
as onObjectSave trigger on Source::B field in the layout.
There is other way that not placing Source::B field, setting it through script...
s 1 2 3
d 1 2 3
the 123 in d file comes from 123 of the source file.
Id like to change 123 of the S file in 456 and automatically the d file changes to 456-
You should use keys in both tables; then create your value list from the primary key and the human-readable datum of the source table, and use that to enter the key into the foreign key field of the target table.
Now the target record has a reference to the source record, and you can display that source data however you like – as long as you don't change the keys.