In order to do that, the serial number would need to be the Key field in at least one of the tables. Then you would make a relationship between the tables based on a new field called [Serial number mirror]. The formula for that field would be [Serial number]
Then on the parent table, you would be able to summarize the # of children and if > 0, then put up a big red error message saying that the serial is not unique enough.
Then on the parent table create a formula checkbox field with a value of TRUE. call id [Serial exists in Table A]
Look up that checkbox field to the child table.
Then put up a big red error message if that lookup field is checked.
