$query1 = 'select Name from Customers where CustId = :custid';
$query2 = 'select Name from Sales where SalesId = :salesid';

$sth1 = $dbh->prepare($query1);
$sth2 = $dbh->prepare($query2);

if ($x > 10) {
  $sth1->bind_param('custid', $custid);
  $sth1->execute();
  while ($row = $sth1->fetchrow_hashref) {
    ...
  }
} else {
  $sth2->bind_param('salesid', $salesid);
  $sth2->execute();
  while ($row = $sth2->fetchrow_hashref) {
    ...
  }
}