I did this with a config/domains.config.php and different folders in the theme directory
<?php
$CONFIG['trusted_domains'] = array (
0 => 'domain1',
1 => 'domain2'
);
if (isset($_SERVER['HTTP_HOST'])) {
switch ($_SERVER['HTTP_HOST']) {
case 'domain1':
$CONFIG['theme']='domain1';
$CONFIG['mail_from_address'] = 'address';
$CONFIG['mail_domain'] = 'domain1';
break;
case 'domain2':
//same for domain2
}
}