validating data for 2 field at the same time

Hi All,
It is possible to restrict data entry for 2 fields at the same time?,
i know how this may sound for you, please don't insult me (just
kidding, this is a great newsgroup). In my first Access database I
just found a big design mistake. There is a table - tblsystem- with 2
fields -upperprinter and lowerprinter- this 2 fields contains the same
type of data, they store the printers serial numbers. Now I found
duplicate entries for the printers, meaning a printer can only belong
to a system but a system has 2 printers, even when I set the no
duplicates property for each of these fields at the table design
level, a printer serial number may be the unique entry for the upper
printer and this same serial number may be the unique entry for the
lower printer, causing the same printer appear in 2 different systems
at the same time. Before I go and change the design, I would like to
quick fix it by creating a rule when data is entered in any of these
fields to check is the serial number already exist in the table-
upperprinter or lowerprinter fields - and not allow the entry if a
system already had the serial number in the upperprinter or lower
printer field.

I will appreciate any help. Thank so much.