The following setup has 2 masters and 2 slaves. The two master will also be used as slaves. The setup will prefer master1 as the master, and switch to master2 if master1 fails.
<?php
$databases['default']['master1'] = array (
'database'=> 'mydatabase',
'username'=> 'readwrite',
'password'=> 'readwrite',
'host'=> 'master1',
'port'=> '',
'driver'=> 'mysql',
'prefix'=> '',
);
$databases['default']['master2'] = array (
'database'=> 'mydatabase',
'username'=> 'readwrite',
'password'=> 'readwrite',
'host'=> 'master2',
'port'=> '',
'driver'=> 'mysql',
'prefix'=> '',
);
$databases['default']['slave'][] = array (
'database'=> 'mydatabase',
'username'=> 'readonly',
'password'=> 'readonly',
'host'=> 'slave1',
'port'=> '',
'driver'=> 'mysql',
'prefix'=> '',
'readonly'=> TRUE,
);
$databases['default']['slave'][] = array (
'database'=> 'mydatabase',
'username'=> 'readonly',
'password'=> 'readonly',
'host'=> 'slave2',
'port'=> '',
'driver'=> 'mysql',
'prefix'=> '',
'readonly'=> TRUE,
);
$databases['default']['slave'][] = $databases['default']['master1'];
$databases['default']['slave'][] = $databases['default']['master2'];
$databases['default']['default'] = array (
'driver'=> 'autoslave',
'master'=> array('master1', 'master2'),
'slave'=> array('slave'),
);
?>