At present OBS Studio only supports a single destination server. It is planned to allow multiple outputs in a future version (as I understand it). This will require additional bandwidth once it is available, but should be able to send the same output stream to each destination, no notable system load increase.
Restream.io is a workaround, but please remember to NOT give them your primary streaming key if possible as they are a third-party; Twitch allows the creation of independent 'guest keys' that the master key is able to revoke in the event of abuse, for example.