Do it as one SQL update statement.
If both are always null, this will work.
If not then split it up accordingly.
NOTE: The dots after the ] bracket are critical.
Its not going to work without them.
This was tested in msAccess 2003
[SELECT Col2 FROM tbl3Columns WHERE Col1=’A’; ]. AS [tblSourceCol2 ],
[SELECT Col3 FROM tbl3Columns WHERE Col1=’B’; ]. AS [tblSourceCol3 ]
SET tbl3Columns.Col2 = tblSourceCol2.Col2, tbl3Columns.Col3 = tblSourceCol3.Col3
WHERE tbl3Columns.Col2 IS NULL;</pre>