First of all, if you use a MySQL server it's typically better and faster to connect via DBD::mysql rather than via ODBC.
Second, you'll want to use MySQL-proxy or a similar solution as a 'load balancer' for your MySQL cluster.
That way from OTRS you connect to only one 'virtual' server.
If you'd define two data backends all queries typically hit all cluster database nodes which seems very inefficient to me.
With the LDAP example you gave the 'failover' happens at connection time.
Within mod_perl typically the connection is not set up for each query but only once and then is kept open by Apache::DBI.