This seems to be not generally true. MysqlDB > 5.5 and MariaDB > something less than 5.3 support index_merge in their query plans. Skimming the mysql page, this

This seems to be not generally true. MysqlDB > 5.5 and MariaDB > something less than 5.3 support index_merge in their query plans. Skimming the mysql page, this allows mysql to evaluate different parts of the where-clause on different indexes and merges the results later on. Most examples on the mysql-man page also use just a single table in their queries.