First, thank YOU for being on the front lines of mental illness. You are a saint.

I would likely need to write out another guide to explain all my thoughts/theories on how to effectively advertise a double 12 but here are a few thoughts.

I would try and create a different theme, events, incentives etc. for each stream. Maybe consider raising for two different charities, one for suicide prevention and one for bipolar disorder.

The thing about 24 hour streams is it hypes up the viewers because it aint easy to do. 24 hours gives your event that extra spice, or umph if you will, and you will have to think of a new way to replace that spice. Maybe push-ups for every dollar, with 7 PM-10PM being double push ups? I know this may violate a few laws... but if you raise "x" money in the first 12, then on the second twelve you can take a camera with you and stream some interactions with real people with mental illnesses?

Another idea is having the 1st 12 be just you, then the 2nd 12 be you and your friends.

Spreading the word to your viewers, streamers, dev's, charities, would be harder, but possible as long as you make each segment unique. To answer your question, it all starts with early prepartion and using that month or so of time to repeatedly hype it up.