Modify the file, sites/all/modules/date/date_elements.inc
function date_combo_validate(), maybe line 521,
after the code
if (empty($errors)) {
add these:
if ($to_date_empty) {
$item[$to_field] = null;
}
sites/all/modules/date/date.theme
function theme_date_display_combination(), maybe line 126
before the code:
if (empty($date1) && empty($date2))
add these :
if (empty($element['#node']->node_data_field_date_1_field_date_1_value2)) {
$date2 = null;
}