Open your table in design view.
Open the Properties box (View menu.)
Set the Validation Rule in the Properties box to:
([Field1] Is Not Null) OR ([Field2 Is Null)
The rule is satisfied in Field1 has a value.
If it doesn't, the only way the rule is satisfied is if Field2 is blank.
Because you are comparing fields, you must use the rule for the table (in
the Properties box), not the rule for a field (in the lower pane of table
design.)
In general, you try to avoid creating a data structure where one field is
dependent on another.
