Bundled channel subs
Streamers would be able to offer a bundle where you sub to 2 or more channels at once for some small discount on full subs, or those channels split the revenue depending on how they setup the split, obviously channels involved would have to agree to be in the bundle.
A streamer and their alt
A streamer and their partner
A streamer and another streamer they want to support
Org or twitch team members
As it would be one viewer buying, the cost to twitch shouldnt be higher as the viewer still only has one pair of eyes. The idea would be to get the viewer to pick up an extra channel that they might only watch a little.