The PeoleCode below will prevent users from inserting more than 2 rows on a grid. The "+" button will disappear the moment a 2nd row is added by a user. I have place this code on a record field under rowinit event.
If GetRowset().ActiveRowCount = 1 Then
GetRowset().InsertEnabled = True;
Else
GetRowset().InsertEnabled = False;
End-If;