mysql> create table test(id int auto_increment primary key, low bigint, high bigint, txt text);
Query OK, 0 rows affected (0.34 sec)

mysql> create index k1 on test (low, high);
Query OK, 0 rows affected (0.94 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select * from test where 123456 between low and high;
+----+-------------+-------+-------+---------------+------+---------+------+------+-----------------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                 |
+----+-------------+-------+-------+---------------+------+---------+------+------+-----------------------+
|  1 | SIMPLE      | test  | range | k1            | k1   | 9       | NULL |    1 | Using index condition |
+----+-------------+-------+-------+---------------+------+---------+------+------+-----------------------+
1 row in set (0.08 sec)