The following setup will preferably use master1 as master and master2 as slave. If one of them fails, it will use the remaining db as both master and slave retaining write capabilities.
<?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']['default'] = array (
'driver'=> 'autoslave',
'master'=> array('master1', 'master2'),
'slave'=> array('master2', 'master1'),
);
?>