Create "Dual sub" emotes
Have Twitch Partners be able to officially partner with each other and create a set of emotes that viewers can only access if they sub to both channels. Each Partner still has their normal emote set of course, but if you sub to both channel A and channel B you also get a few "AB" exclusive emotes. Unsubbing from either loses you access to the dual sub emotes.
Many pairs of Partners already have emotes of their own that combo well with each other, purposely. For example, two Partners have "wave" emotes that face each other to look like a high five, etc.
This will encourage collaboration more and can be extended to Twitch Teams as well (Sub to every Partner on the team and get team-specific emotes). Don't limit each Partner to just one "sub partner" either, let the community control who partners with who. I believe this will drive a higher sub count overall and help discoverability for friends on a higher level than just raids and shoutouts.
now that Twitch just announced Affiliates, in the future, expand this to allow Partners to team with Affiliates for a dual emote. This will encourage the hell out of collaboration not just between Partners but all across Twitch.