MSM site on different domains (NOT subdomains)

When logged in to CP we can switch sites. When logged in through a frontend form we can not.

Settings for all sites: cookies cookies

Config system:

    $config['allow_multi_logins'] = 'y';
$config['multi_login_sites'] = "http://www.domain_1.org/|http://www.domain_2.org/";

settings in each sites index . php

    $assign_to_config['allow_multi_logins'] = 'y';
$assign_to_config['multi_login_sites'] = "http://www.domain_1.org/|http://www.domain_2.org/";

We do hope to get some advise here, but if this old answer is still true we must accept this situation: Registering/Logging into multiple sites at a time with MSM

Thanks a lot for answering!